Everyone who uses a computer has been at one time or another indebted to antivirus software. It sits like a watchdog in your PC's backyard, waiting for some pesky virus to jimmy the lock and try to mess with the treasures you've stored in the inner sanctum of your hard drive. And, like a high-strung Doberman, an antivirus is much more useful to you if you understand how it operates, when to feed it, how often to walk it, et cetera.

Firstly, a few words on viruses. A virus is more than just a program that can do bad things to your computer, such as damage your files or erase them altogether or make it impossible for you to access them. A virus is a program made up of binary code (ones and zeroes, just like the other programs that run you PC); and the precise order of that code is called the virus' signature. Each virus has a different signature.

Specifically, viral signatures are made up of self-replicating codes. No clean computer program automatically replicates -- a fact that gives antivirus programs something to look for. Antivirus programs learn and memorize all the different replicating viral signatures that are already out there (40,000 to date), and then compare the binary signatures of incoming files against those already-known viral signatures to see if anything suspect is lurking on your doorstep.

Another weapon possessed by the average antivirus is something called heuristics. No, this is not a newfangled religion, nor is it a new-age euphemism for hysteria; it's a technology that allows antiviral programs to detect possible variants of the 40,000 known viruses. The drawback to this technology is that it is not 100% accurate. Sometimes what resembles a viral variant is actually a safe program. In this case, the user is often forced to delete a file for no reason.

There is another type of virus, however, which is more insidious than your run-of-the-mill piece of malicious code. It's called a polymorphic virus, and its binary signature changes constantly. Antiviruses deal with these little buggers by opening them not in the general space of the computer, but within the antiviral application itself. The potential polymorphic virus is allowed to do its stuff while the antivirus looks on. If foul programming is suspected, the user is alerted to the virus and given the option of deleting it; if not, the program is allowed to run its course outside the antivirus application.

The most common methods of transmitting viruses include downloads from the internet, files saved from floppy disks, and e-mail attachments. It's a myth that you can contract a virus just by reading an e-mail message. You must run a program -- i.e., open an attachment, or floppy file, or anything that executes a set of code -- in order to potentially kick-start a virus, so merely reading e-mail is an entirely safe proposition. In addition, certain antiviruses are installed on your e-mail server. These particular watchdogs don't even tell you when they spot a virus; they just dispose of it without notifying you. In today's world of service-oriented antivirus companies, this is becoming the favoured method of dealing with bugs. The customer is kept both protected and calm. After all, what you don't know can't hurt you as long as your antivirus knows about it.