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.
What is a worm?
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:
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.
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:
- Trojan horses
- 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:
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:
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:
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:
Windows XP, Windows Vista, or Windows 7
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
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
Windows Vista and Windows 7
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:
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.
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:
Windows XP, Windows Vista, and Windows 7
- Click the Start button, 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 OK.
- Click the Start button, 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
- Click OK.
How to obtain computer virus and security-related support
For United States
Want to chat with a live person? Our Answer Tech trained professionals are ready to help:
For United States and Canada
Help protect your computer from viruses and malware:
Virus Solution and Security Center
Get help with installing updates:
Support for Microsoft Update
Security solutions for IT Professionals:
TechNet Security Troubleshooting and Support
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.
Article ID: 129972 - Last Review: Apr 23, 2015 - Revision: 1