There is alot of fiction and hype out there about computer viruses. Not everything you hear is true. In fact, most e-mail messages that circulate about viruses are a special version of urban legends. Remember that before you forward that next e-mail about a virus warning! A good source of information about this is Urban Legends and Folklore - Home Page.
Despite the hype and fiction, computer viruses are real and can cause problems, sometimes serious problems. Therefore, it is a good idea to have a virus detection program on your system. I recommend McAfee VirusScan (not the online version) or Norton Anti-Virus. I do NOT recommend using all the other tools McAfee and Norton want you to install with it. It has been my experience that the crash guard programs, etc. cause more problems than they solve. Just my humble opinion, take it or leave it, you might have had a different experience with the software.
How do you separate the fact from fiction? To find accurate virus information, visit McAfee Anti-Virus, particularly the Virus Information Library and the Virus Hoaxes. I believe Norton also has some good virus pages and I will add these later when I have time to look them up (or when someone e-mails them to me!).
Viruses also have to come from somewhere - such as disks, computer CD's, or a network, including the Internet. Again, generally, an infected program has to run to start the virus on your system. Be cautious about new disks and downloaded programs. Do not be paranoid, just cautious. How well do you trust the source? This isn't fool-proof. I'm cautious and thus a pretty good source, but I've recently been the victim of spreading a virus. You can be cautious and still get a virus. All you can do is minimize the chances. The only way to never get a computer virus is to pretty much never use a computer. But, you can minimize your chances with some education.
Also, be very cautious about generic e-mails. If the e-mail doesn't look very personalized, it could be a virus-generated e-mail. If the e-mail language doesn't sound like the person it came from, be cautious!