We often receive mail, such as this one, from readers who think they've been hacked: "Icons on my desktop are moving around whenever I use my mouse." In a sense, these hapless folks have been hacked, but not by a malicious user. Instead, a virus has broken into the machine. A virus can install code that captures your credit card information or open a hidden back door and allow others to come and go inside your computer without your knowledge. As more and more people gain access to the Internet, viruses grow more common.

Generally, the terms virus  and worm  can be used interchangeably, though it is important to understand the differences. Briefly, a virus is code that attacks a specific application. Worms, on the other hand, are not specific to one program and carry their infected code from computer to computer. For example, Melissa is a virus that specifically infects the Microsoft Word file normal.dot, while the I Love You worm doesn't infect any files. Both spread by sending copies of themselves via e-mail.

The big viruses
The mass-mailing worm SirCam randomly chooses a document from an infected computer and sends it out--in some cases, e-mailing private information to the world at large. In general, these worms aren't destructive, although a few, such as Badtrans.B, not only spread private password information on the Internet, but also deleted files along the way. Even more destructive worms, such as Magistr, may wait several weeks before deleting your entire hard drive.

Antivirus help
Fortunately, many antivirus options exist. Current technology favors pattern matching, which means that your antivirus program stops viruses by matching them to definitions in its database. This is why you should update your antivirus app regularly. A few antivirus programs automatically download the latest versions of the pattern files. Some programs also include heuristic software, which applies general virus rules to stop potential new viruses that are not defined in its pattern-matching database. Antivirus software that will protect you are  Computer Associates EZ Antivirus, McAfee VirusScan, PCCillin, Panda, Antivirus Norton AntiVirus, just mention a few.