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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:

  • Viruses

  • Worms

  • Trojan horses

  • Spyware

  • Rogue security software

Select a heading below for more information 

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.

Tip: 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.

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.

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.

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:

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.

For more information see Protect yourself from tech support scams. 

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

1. 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 Windows Update: FAQ

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

  2. In the results area, click Windows Update.

  3. Click Check for Updates.

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

2. 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.

3. 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

4. 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:

  1. Note the name of the rogue security software. For this example, we'll call it XP Security Agent 2020.

  2. Restart your computer.

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

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

    Tip: Safe Mode starts Windows with only the minimum number of drivers and services necessary for operation. This often prevents malware or other misbehaving software from loading and makes it easier to clean them out.

  5. 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.

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

  7. Click the Shortcut tab.

  8. 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 2020.

    Note: The folder name is frequently a random number or string of characters.

  9. Click Open File Location.

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

  11. Scroll until you find the rogue security software program folder. For example, XP Security Agent 2020.

  12. Right-click the folder, and then click Delete.

  13. Restart your computer.

  14. Go to the Microsoft Safety Scanner website.

  15. Click the Download Now button, and then click Run.

  16. 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.

5. Run Microsoft Defender Offline

Microsoft Defender Offline is an anti-malware tool that helps remove difficult to eliminate viruses that start before Windows starts. Starting with Windows 10, Microsoft Defender Offline is built in. To use it follow the steps in this article: Help protect my PC with Microsoft Defender Offline.

  1. On an uninfected computer, go to Help protect my PC with Microsoft Defender Offline.

  2. 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, see Is my PC running the 32-bit or 64-bit version of Windows.

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

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

  5. 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.

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

Once malware is running in your computer's memory it can be harder to remove. The malware is often created with what we call "persistence" which means that if it's running and sees that its files have been deleted or quarantined by security software, the malware will attempt to re-write the files to your drive.

Restarting gives us a chance to load Windows without loading the malware, so we can clean the files off the drive without the malware in memory attempting to reinstall itself.

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

Confirm that the Windows firewall is turned on. See Turn Microsoft Defender Firewall on or off for instructions on how to do that on modern versions of Windows.

  1. Click the Start button, and then click Control Panel.

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

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

  4. Under each network location, click Turn on Windows Firewall, and then click OK.

Keep your computer up to date

For more information about how to set Automatic Updates in Windows, see Windows Update: FAQ

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 sites 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. An administrator account can access anything on the system, and any malware run with an administrator account can use the administrator permissions to potentially infect or damage any files on the system.

For more information about how to protect a computer against viruses, see Protect my PC from viruses.

How to obtain computer virus and security-related support

In the United States:

Want to chat with a live person? Our Answer Tech trained professionals are ready to help:

Answer Desk

Security information and training

Microsoft Security Help and Learning

Security solutions for IT Professionals:

Microsoft Security and Response Center

For locations outside North America:

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

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