What is a virus?

A virus is a piece of computer code that attaches itself to a program or file so it can spread from computer to computer, infecting as it travels. Viruses can damage your software, your hardware, and your files.

Virus (n.) Code written with the express intention of replicating itself. A virus attempts to spread from computer to computer by attaching itself to a host program. It may damage hardware, software, or information.

Just as human viruses range in severity from Ebola to the 24-hour flu, computer viruses range from the mildly annoying to the downright destructive. The good news is that a true virus does not spread without human action to move it along, such as sharing a file or sending an e-mail.

What is a worm?

A worm, like a virus, is designed to copy itself from one computer to another, but it does so automatically by taking control of features on the computer that can transport files or information. Once you have a worm in your system it can travel alone. A great danger of worms is their ability to replicate in great volume. For example, a worm could send out copies of itself to everyone listed in your e-mail address book, and their computers would then do the same, causing a domino effect of heavy network traffic that would slow down business networks and the Internet as a whole. When new worms are unleashed, they spread very quickly, clogging networks and possibly making you wait twice as long for you (and everyone else) to view Web pages on the Internet.

Worm (n.) A subclass of virus. A worm generally spreads without user action and distributes complete copies (possibly modified) of itself across networks. A worm can consume memory or network bandwidth, thus causing a computer to stop responding.

Because worms don't need to travel via a "host" program or file, they can also tunnel into your system and allow somebody else to take control of your computer remotely. Recent examples of worms included the Sasser worm and the Blaster worm.

What is a Trojan Horse?

Just as the mythological Trojan Horse appeared to be a gift, but turned out to contain Greek soldiers who overtook the city of Troy, today's Trojan Horses are computer programs that appear to be useful software, but instead they compromise your security and cause a lot of damage. A recent Trojan Horse came in the form of an e-mail that included attachments claiming to be Microsoft security updates, but turned out to be viruses that attempted to disable antivirus and firewall software.

Trojan Horse (n.) A computer program that appears to be useful but that actually does damage.

Trojan Horses spread when people are lured into opening a program because they think it comes from a legitimate source. To better protect users, Microsoft often sends out security bulletins via e-mail, but they will never contain attachments. We also publish all our security alerts on our Security Web site before we e-mail them to our customers.

Trojan Horses can also be included in software that you download for free. Never download software from a source that you don't trust. Always download Microsoft updates and patches from Microsoft Windows Update or Microsoft Office Update.

How can I tell if I have a worm or other virus?

When you open and run an infected program, you might not know you've contracted a virus. Your computer may slow down, stop responding, or crash and restart every few minutes. Sometimes a virus will attack the files you need to start up a computer. In this case, you might press the power button and find yourself staring at a blank screen.

All of these symptoms are common signs that your computer has a virus, although they could also be caused by hardware or software problems that have nothing to do with having a virus.

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

Unless you have up-to-date antivirus software installed on your computer, there is no sure way to know if you have a virus or not. If you don't have current antivirus software or if you're interested in installing a different brand of antivirus software, read our tips for reducing your virus risk.