What is a computer virus?

An introduction to computer viruses: Part 1

Computer viruses are software programs deliberately designed to interfere with computer operation, record, corrupt, or delete data, or spread themselves to other computers and throughout the Internet, often slowing things down and causing other problems in the process.

Just as human viruses range in severity from the 24-hour flu to the Ebola virus, computer viruses range from the mildly annoying to the downright destructive, and come in new and different forms. The good news is that with an ounce of prevention and a little knowledge, you are less likely to fall victim to viruses and you can diminish their impact.

How do viruses work?

Basic viruses typically require unwary computer users to inadvertently share or send them. Some viruses that are more sophisticated, such as worms, can replicate and send themselves automatically to other computers by controlling other software programs, such as an e-mail sharing application. Certain viruses, called Trojans (named after the fabled Trojan horse), can falsely appear as a beneficial program to coax users into downloading them. Some Trojans can even provide expected results while quietly damaging your system or other networked computers at the same time.

Although it's good to be aware of these different types of viruses and how they work, what is most important is that you keep your computer current with the latest updates and antivirus tools, stay current about recent threats, and that you follow a few basic rules when surfing the Internet, downloading files, and opening attachments. Once a virus is on your computer, its type or the method it used to get there is not as critical as removing it and preventing further infection.

How do I know if a virus has infected my computer?

An introduction to computer viruses: Part 2

After you open and run an infected program or attachment on your computer, you might not realize that you've introduced a virus until you notice something isn't quite right.

Here are a few primary indicators that your computer might be infected:

These are common signs of infection—but they might also indicate hardware or software problems that have nothing to do with a virus. Unless you run the Microsoft Malicious Software Removal Tool and install industry-standard, up-to-date antivirus software on your computer, there is no way to be certain if your computer is infected with a virus or not. If you don't have current antivirus software installed or if you're interested in installing a different brand, visit Security software downloads page for more information

Tip: Beware of messages warning you that you sent e-mail that contained a virus. This can indicate that the virus has listed your e-mail address as the sender of tainted e-mail. This does not necessarily mean you have a virus. Some viruses have the ability to forge e-mail addresses

How do I remove a virus?

An introduction to computer viruses: Part 3

Even for an expert, removing a virus properly from a system is often a daunting task without the help of specific tools designed for the job. Some viruses and other unwanted software (including spyware) are even designed to reinstall themselves after they have been detected and removed! Fortunately, through updating your computer and using free, trial-period, or low-cost antivirus tools offered by many companies, you can help permanently remove (and prevent) unwanted software.

Steps to help remove a virus:

Why didn't my antivirus software work?

If you already have antivirus software installed on your system, it's very important to keep it current on a regular basis with the latest updates (usually called definition files) that help the tool identify and remove the latest threats. In addition, not all antivirus tools are created equal; if you find that the one you use isn't working to your satisfaction, you should do some research and try an alternative.

How do I install updates and antivirus software if I can't use my computer?

In some cases, it might be difficult to download tools or update your system if a virus has already disabled your computer, creating a Catch-22. In this case, use a friend's or other computer to download the tools to a disk, or contact Microsoft Services support for additional help.

How do I help protect my computer against viruses?

An introduction to computer viruses: Part 4

Nothing can guarantee the security of your computer 100 percent. However, you can continue to improve your computer's security and decrease the possibility of infection by keeping your system up-to-date, maintaining a current antivirus software subscription, and following a few best practices.

Tip: Because no security method is guaranteed, it's important to back up critical files on a regular basis before you encounter a virus or other problems.

Steps to help avoid viruses: