Myths about computer viruses abound. For example, many people think viruses are contracted primarily through a modem, while downloading software. In reality, a study by the National Computer Security Association and Dataquest found that floppy disks are by far the most frequent carriers. Sixty-five percent of all viruses in the study were contracted through floppies; local-area networks spread another 25%. Only 7% of viruses were admitted by modem. Inc. asked Robert Slade, author of Robert Slade's Guide to Computer Viruses (Springer-Verlag, New York City, 1994, $29), to set the record straight.

What exactly is a computer virus -- and what can it do to my computer? A virus is any program that reproduces itself by using the resources of your computer without your knowledge or consent. Most viruses are not intentionally malicious. For example, Stoned, a common virus, doesn't seem to have been intended to have ill effects. But because its author wasn't familiar with certain disk formats, Stoned can hide files on some disks. A truly malicious virus -- say, one that erases an entire hard disk -- doesn't have much chance to spread because it destroys its "host." But, in all likelihood, any virus you get will eventually conflict with something on your system.

How common are viruses? It's hard to say, because they often go unreported. In my book I estimate that about a quarter of computer users will deal with a virus within the next two years. But that's a guess. There are about 6,500 known viruses, and new ones are reported every month.

What are the biggest misconceptions? There's a stigma attached to viruses; people think they have something to do with pirated software. People also think, "If I don't have a modem, I'm safe." And people think it can't happen to them. None of those three things is true.

How should I protect my computer system? First of all, back up regularly. Second, everybody should have antiviral software; there's no excuse not to get it. I think shareware is often better than the commercial products.

How often should I check for viruses? Check every disk and new program you receive, including Windows data files. Some antiviral software can monitor your system automatically, either all the time or at regular intervals. Don't forget to keep your antiviral software up-to-date.

What do I do if I discover my computer has a virus? Don't panic. You can remove most viruses by running good antiviral scanning software, which should have a "disinfecting" feature. If you can't fix the problem easily, confirm that you have a virus by using a second brand of antiviral software; it's possible to get a "false positive." If necessary, get rid of an infected file and reinstall it from your backup. Whatever you do, be sure to scan all your diskettes with the antiviral software -- or you may well quickly reinfect your computer.