Where do computer virus myths come from? It's hard to say. One minute you think you know how they work, and the next you're being told you can get viruses from cookies, Javascript on Web pages, viewing GIF files, and so on. All nonsense of course, but it's understandable to feel concerned. And it's a lot healthier than completely assuming you're safe from computer viruses, because unless you get rid of your PC that certainly isn't true.

Definitions

A virus is a computer program that is designed to enter PCs without the user's knowledge, then replicate itself throughout the system. Some viruses carry a ‘payload' which activates eventually – this can be anything from displaying a silly message on the screen to wiping out all the files on your hard disk.

An important step to protecting yourself is to realize when programs are running on your PC, because viruses can exploit them all. The first opportunity occurs when you switch on your computer. Both floppy disks and hard disks contain a little ‘boot sector' program, which either displays a ‘you can't boot from me' message (on a normal floppy), or loads your operating system (on a hard disk). Boot sector viruses infect this program, so loading into memory every time you start your PC, then copying itself to every floppy disk you read.

Other viruses target files containing program code; every time you run the infected program, it loads itself into memory, then might copy itself to other programs. Not just .exe files, either - .com, .dll, .ocx, .drv, .sys, and so on, are all at risk. After copying itself the original application continues to run and you never realize there's a problem, until the payload hits you.

Still, at least data files are safe, right? Wrong. Many Microsoft programs enable files such as Word documents and spreadsheets to contain program code (macros). This is great for automated office applications, but also presents another risk point. Just viewing a document carrying a macro virus can be enough to infect your whole system.

Don't Panic

Alarmed? You should be, but there's no need to panic. Modern antivirus software has some powerful features to prevent you getting infected. Two methods are particularly common.

The first involves maintaining a table of virus signatures – sequences of bytes that occur in the virus program itself. Scan a downloaded file, and if you find that signature then you know the file might be infected. The signature approach is useful and quick, but can give false alarms if the signature occurs innocently somewhere. It's no use on new viruses that aren't in the signature file yet. And some viruses are polymorphic, meaning they change their own structure and just don't have a consistent signature anyway.

Fortunately the best antivirus software has another weapon in its arsenal – the heuristic approach. This involves looking for virus-like behavior – writing to an executable file, perhaps, or copying information to the boot sector. It's a clever idea, and can even help detect completely new viruses.

Protect Yourself

Installing good antivirus software is a great idea, but it's only the first step. There are lots of other things you can do to help protect yourself. First you need to keep the software updated to cope with new threats. Some antivirus programs can update themselves automatically, but it's still worth visiting the author's web site every week to find out what's going on. And don't forget about the security settings on other software.

There will always be some risk from viruses, but good antivirus software, combined with regular backups, should mean you can recover, if the worst happens.