How to prevent and remove viruses and other malware

What is a computer virus?

A computer virus is a small software program that spreads from one computer to another and interferes with computer operation. A computer virus might corrupt or delete data on a computer, use an email program to spread the virus to other computers, or even delete everything on the hard disk.

Computer viruses are frequently spread by attachments in email messages or by instant messaging messages. Therefore, you must never open an email attachment unless you know who sent the message or you are expecting the email attachment. Viruses can be disguised as attachments of funny images, greeting cards, or audio and video files. Computer viruses also spread through downloads on the Internet. They can be hidden in pirated software or in other files or programs that you might download.

Symptoms of a computer virus

For information about the symptoms of a computer virus, go to the
Microsoft PC Security website.

A worm is computer code that spreads without user interaction. Most worms begin as email attachments that infect a computer when they’re opened. The worm scans the infected computer for files, such as address books or temporary webpages, that contain email addresses. The worm uses the addresses to send infected email messages, and frequently mimics (or spoofs) the «From» addresses in later email messages so that those infected messages seem to be from someone you know. Worms then spread automatically through email messages, networks, or operating system vulnerabilities, frequently overwhelming those systems before the cause is known. Worms aren’t always destructive to computers, but they usually cause computer and network performance and stability problems.

What is a trojan horse?

A trojan horse is a malicious software program that hides inside other programs. It enters a computer hidden inside a legitimate program, such as a screen saver. Then it puts code into the operating system that enables a hacker to access the infected computer. Trojan horses do not usually spread by themselves. They are spread by viruses, worms, or downloaded software.

What is spyware?

Spyware can install on your computer without your knowledge. These programs can change your computer’s configuration or collect advertising data and personal information. Spyware can track Internet search habits and can also redirect your web browser to a different website than you intend to go to.

What is rogue security software?

A rogue security software program tries to make you think that your computer is infected by a virus and usually prompts you to download or buy a product that removes the virus. The names of these products frequently contain words like Antivirus, Shield, Security, Protection, or Fixer. This makes them sound legitimate. They frequently run right after you download them, or the next time that your computer starts. Rogue security software can prevent applications, such as Internet Explorer, from opening. Rogue security software might also display legitimate and important Windows files as infections. Typical error messages or pop-up messages might contain the following phrases:

Warning!
Your computer is infected!
This computer is infected by spyware and adware.

Note If you receive a message in a popup dialog box that resembles this warning, press ALT + F4 on your keyboard to close the dialog box. Do not click anything inside the dialog box. If a warning, such as the one here, keeps appearing when you try to close the dialog box, it’s a good indication that the message is malicious.

Are you sure you want to navigate from this page?
Your computer is infected! They can cause data lost and file corruption and need to be treated as soon as possible. Press CANCEL to prevent it. Return to System Security and download it to secure your PC.
Press OK to Continue or Cancel to stay on the current page.

If you see this kind of message, then don’t download or buy the software.

What is malware?

Malware is a term that is used for malicious software that is designed to do damage or unwanted actions to a computer system. Examples of malware include the following:

Rogue security software

How to remove malware such as a virus, spyware, or rogue security software

Removing a computer virus or spyware can be difficult without the help of malicious software removal tools. Some computer viruses and other unwanted software reinstall themselves after the viruses and spyware are detected and removed. Fortunately, by updating the computer and by using malicious software removal tools, you can help permanently remove unwanted software.

For more information about how to remove a computer virus and spyware, see the following article in the Microsoft Knowledge Base:

2671662 — Microsoft resources and guidance for removal of malware and viruses Note If you cannot access the Internet on your computer, use another computer to help you follow the steps in the “How to reset your Internet Explorer proxy settings” section on the computer that may be infected.

To remove a computer virus and other malicious software, follow these steps in order.

Install the latest updates from Microsoft Update

Note A computer virus may prevent you from accessing the Microsoft Update website to install the latest updates. We recommend that you set the Automatic Updates service to run automatically so that a computer is not missing any important updates. For more information, see the following article in the Microsoft Knowledge Base:

Click Start, and then type Windows Update in the search box.

In the results area, click Windows Update.

Click Check for Updates.

Follow the instructions to download and install the latest Windows Updates.

Use the free Microsoft Safety Scanner

Microsoft offers a free online tool that scans and helps remove potential threats from your computer. To perform the scan, go to the Microsoft Safety Scanner website.

Use the Windows Malicious Software Removal Tool

For more information about the Microsoft Malicious Software Removal Tool, see the following article in the Microsoft Knowledge Base:

890830 — Remove specific prevalent malware with Windows Malicious Software Removal Tool
Manually remove the rogue security software

If the rogue security software can’t be detected or removed by using Microsoft Safety Scanner or the Windows Malicious Software Removal Tool, try the following steps:

Note the name of the rogue security software. For this example, we’ll call it XP Security Agent 2010.

Restart your computer.

When you see the computer’s manufacturer’s logo, repeatedly press the F8 key.

When you are prompted, use the arrow keys to highlight Safe Mode with Networking, and then press Enter.

Click the Start button and check whether the rogue security software appears on the Start menu. If it’s not listed there, click All Programs and scroll to find the rogue security software’s name.

Right-click the name of the rogue security software program, and then click Properties.

Click the Shortcut tab.

In the Properties dialog box, check the path of the rogue security software program that is listed in Target. For example, C:\Program Files\XP Security Agent 2010.

Note The folder name frequently is a random number.

Click Open File Location.

In the Program Files window, click Program Files in the address bar.

Scroll until you find the rogue security software program folder. For example, XP Security Agent 2010.

Right-click the folder, and then click Delete.

Restart your computer.

Click the Download Now button, and then click Run.

Follow the instructions to scan your computer and help remove the rogue security software.

If you suspect that your computer is infected with rogue security software that was not detected by using Microsoft security solutions, you can submit samples by using the Microsoft Malware Protection Center submission form.

For more information about rogue security software, go to the Watch out for fake virus alerts website.

Install and run Microsoft Security Essentials or Windows Defender

Microsoft offers a free malicious removal program called Microsoft Security Essentials that helps protect your computer from becoming infected. To install Microsoft Security Essentials, follow these steps:

Click Free Download.

Click Run, and then follow the instructions to install Microsoft Security Essentials.

After installation, restart your computer.

Click Start, click All Programs, and then click Microsoft Security Essentials.

On the Home tab, select the Full scan option, and then click Scan now.

Install Windows Defender Offline

Windows Defender Offline is a malware tool that helps remove difficult to eliminate viruses that start before Windows starts. To use Windows Defender Offline, follow these steps:

On an uninfected computer, go to the What is Windows Defender Offline website.

Click Download the 32 bit version or Download the 64 bit version, depending on which operating system that you are running. If you’re unsure of which operating system that you are running, go to the Is my PC running the 32-bit or 64-bit version of Windows website.

When you are prompted, click Save As, and then save the file to a DVD, CD, or USB flash drive.

On the infected computer, insert the DVD, CD, or USB flash drive, and then restart the computer.

When you are prompted, press a key to select an option to use to start your computer, such as F12, F5, or F8, depending on the kind of computer that you are using.

Use the arrow key to scroll to the drive where you installed Windows Defender Offline file. Windows Defender Offline starts and immediately scans for malware.

For more information about how to remove a computer virus, go to the How do I remove a computer virus website.

How to protect your computer against malware

There are actions that you can take to help protect your computer against malware.

Turn on the firewall

Click the Start button, and then click Control Panel.

In the Search box, type firewall, and then click Windows Firewall.

In the left pane, click Turn Windows Firewall on or off (you may be prompted to enter your administrator password).

Under each network location, click On (recommended) (Windows Vista) or Turn on Windows Firewall (Windows 7), and then click OK.

For more information about how to turn on your firewall in Windows 7, go to the Turn Windows 7 Firewall on or off website.

Keep your computer up to date

For more information about how to set Automatic Updates in Windows, see the following article in the Microsoft Knowledge Base:

306525 — How to configure and use Automatic Updates in Windows
Install Microsoft Security Essentials and keep it up to date

For more information about how to install and use Microsoft Security Essentials, go to the Microsoft Security Essentials website.

Don’t be tricked into downloading malware

Here are some tips that can help protect you from downloading software that you don’t want:

Only download programs from websites that you trust. If you’re not sure whether to trust a program that you want to download, enter the name of the program into your favorite search engine to see whether anyone else has reported that it contains spyware.

Read all security warnings, license agreements, and privacy statements that are associated with any software that you download.

Never click «Agree» or «OK» to close a window that you suspect might be spyware. Instead, click the red «x» in the corner of the window or press Alt + F4 on your keyboard to close a window.

Be wary of popular «free» music and movie file-sharing programs, and make sure that you understand all the software packaged with those programs.

Use a standard user account instead of an administrator account. For more information, go to the Why use a standard account instead of an administrator account website.

For more information about how to protect a computer against viruses, go to the How to boost your malware defense and protect your PC website.

How to reset your Internet Explorer proxy settings

Malicious software might change Internet Explorer proxy settings, and these changes can prevent you from accessing Windows Update or any Microsoft Security sites.

To have us change your Internet Explorer proxy settings for you, click the Fix it button or link. Click Run in the File Download dialog box, and follow the steps in the Fix it wizard:

This wizard may be in English only. However, the automatic fix also works for other language versions of Windows.

If you are not using the computer that has the problem, save the Fix it solution to a flash drive or a CD and then run it on the computer that has the problem.

To change your Internet Explorer proxy settings yourself, follow these steps:

Click Start and then click Run.

In the Run box, copy and paste the following:

reg add «HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings» /v ProxyEnable /t REG_DWORD /d 0 /f

Click Start, and then click Run.

In the Run box, copy and paste the following:

reg delete «HKCU\Software\Microsoft\Windows\CurrentVersion\Internet Settings» /v ProxyServer /f

For more information about how to reset your Internet Explorer proxy settings, visit the following Microsoft Knowledge Base article:

2289942 — How to reset your Internet Explorer proxy settings
How to obtain computer virus and security-related support

For United States
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For United States and Canada
Help protect your computer from viruses and malware:

Get help with installing updates:

Security solutions for IT Professionals:

Support by country:

If you are using a PC running Windows 8, click here for more information about how to help protect your PC from viruses, or click here for more information about how to find and remove viruses.

For locations outside North America
For computer virus and security-related support for locations outside North America, go to the Microsoft Support website.

Troubleshoot problems with detecting and removing malware

The troubleshooting info in this topic might help you if you’re experiencing any of the following problems when detecting and removing malware with Microsoft Defender Antivirus, Microsoft Security Essentials, or other Microsoft antimalware solutions:

If scans are taking too long or appear to be progressing very slowly, consider the following solutions:

Ensure you have sufficient disk space

Run scans while your PC is idle by closing all other programs

Microsoft Defender Antivirus requires disk space to remove and quarantine malware files. It might be prevented from completely removing a threat if there isn’t enough space on your PC, particularly on your system drive (commonly drive C). See the following to help free up space:

After you’ve freed up some space, update and then run a scan again.

In general, full scans can take a long time if you have a large disk with lots of files. Large files, especially archives such as ZIP files, take longer to scan.

If Microsoft Defender Antivirus continually encounters errors during scans or during malware removal, try the following solutions:

Please provide feedback to us, so we can deliver fixes as fast as possible. By default, Windows automatically collects error information, but describing the error on the Feedback Hub app can help us address the error more efficiently.

Tip: You can quickly launch the Windows Feedback Hub app in Windows 10 by pressing the Windows logo key + F.

Run Windows Update to apply any fixes and ensure you have the latest components.

If Microsoft Defender Antivirus continually encounters errors during updates, try installing the latest protection updates manually.

To detect the latest threats, use a robust antimalware product, like Microsoft Defender Antivirus, which is built into Windows 10 and Windows 8.1. Ensure that critical security features are turned on and that Microsoft Defender Antivirus is fully updated before scanning.

Use Microsoft Defender Antivirus with cloud-based protection

By default, the following advanced features are enabled. If you’ve turned them off, you should enable them for the best protection:

Automatic sample submission

To turn on these features:

Select Start > Settings > Update & Security > Windows Security > Virus & threat protection.

Under Virus & threat protection settings, select Manage settings.

Make sure the settings for Cloud-delivered protection and Automatic sample submission are turned On.

These settings significantly increase the chances of detecting never-before-seen malware and enable the automated creation of new protection updates that help immunize all other computers running Microsoft Defender Antivirus from the newly discovered threats.

Update Microsoft Defender Antivirus before scanning

By default, Microsoft Defender Antivirus updates definitions automatically at least once every day. You can also manually check for updates:

Select Start > Settings > Update & Security > Windows Security > Virus & threat protection.

Under Virus & threat protection updates, select Check for updates.

Under Threat definitions, select Check for updates.

If you continue to encounter suspicious files that are not detected by Microsoft Defender Antivirus, submit the files to Microsoft for analysis.

Even after a malware has been removed, it might come back if you visit the website that hosts it or receive it again by email. Avoid websites that might contain malware, such as sites that provide illegal downloads.

To block threats from malicious websites, use a modern browser like Microsoft Edge, which uses Microsoft Defender SmartScreen to identify sites with poor reputation. Upgrade to the latest version of Windows to benefit from a host of built-in security enhancements.

In some cases, redetection of the same malware is due to an undetected malware component constantly, quietly, reinstalling the detected malware. The malware is typically reinstalled, and redetected, right after you restart your PC. To resolve this, try scanning with Microsoft Defender Offline to catch hidden threats

Scan with Windows Defender Offline

If the same malware keeps infecting your PC, use Windows Defender Offline to look for and remove recurring malware. Microsoft Defender Offline is a scanning tool that works outside of Windows, allowing it to catch and clean infections that hide themselves when Windows is running.

Note: Before initiating a Microsoft Defender Offline scan, make sure you’ve saved your work. Your PC will restart before starting the scan.

To start an offline scan in Windows 10:

Select Start > Settings > Update & Security > Windows Security > Virus & threat protection.

Under Current threats, select Scan options.

Select Windows Defender Offline scan and then select Scan now.

On Windows 8.1 you will need to download Microsoft Defender Offline as a separate tool. For more information, see Help protect my PC with Microsoft Defender Offline.

If malware has caused irreversible changes to your PC, you can try to reset your PC. This might involve restoring data from backup.

Reset, restore, or reinstall your PC

Back up any files and settings you want to keep so that you can restore them later. Windows provides several options on how you can reset or refresh your PC. If you choose to manually reinstall, you will need to prepare installation discs, product keys, and setup files.

Note: Whenever possible, restore your files from backups generated before the infection and stored in an external location, such as OneDrive, which provides regular cloud-based backups with version histories. Backups that are on your PC during an infection might have already been modified by the malware.

See the following articles for more information about reinstalling or recovering Windows:

As soon as you restore your PC, make sure you have the latest software running. The latest versions of software include available fixes of known security issues. This will help ensure your PC is not infected by malware that exploit security vulnerabilities.

See the following articles for more information about updating Microsoft software and third-party applications:

Provide feedback to Microsoft

Microsoft continually works on enhancing the user experience on all current products, including Windows Defender Antivirus. We encourage all customers to make use of the following feedback channels included in Windows 10:

Set Windows to automatically prompt for your feedback. Windows is already configured to automatically prompt for feedback by default. To ensure this feature is turned on, select Start > Settings > Privacy > Diagnostics & feedback. Under Feedback frequency, make sure that Windows is set to ask for your feedback automatically.

Manually send feedback at any time through the Feedback Hub app. To send feedback, type Feedback Hub in the search box on the taskbar, then select it from the list of results to open the app. In the app, select Feedback > Add new feedback. Select Security, Privacy, and Accounts > Windows Defender Antivirus as the category.

Read Diagnostics, feedback, and privacy in Windows 10 for questions about privacy and feedback settings.