The best defence against viruses is education.
1 - Turn off the preview pane in your email reader. Many viruses come in via email and will automaticly run when you look at the email. It's like getting sent straight to jail with out passing go. You don't get a chance. It will also cut down on the spam you receive.
2 - Do not open every attachment that you get. Delete emails from people you don't know with out opening them (this will also cut down on spam).
3 - Turn off hiding of known file extensions (this is important!). Many viruses come via email with names like mypicture.jpg.bat. When you have the known file extensions hidden you don't see the .bat and think it's a picture file. Microsoft enables hidden extensions by default (it's almost like Microsoft WANTS you to get viruses).
4 - Get a good antivirus, but don't rely on it to protect you. Think of antivirus like the flu shot from the dr. When you get a flu shot from the Dr it protects you from a certain number of strains of flu but there are new flu viruses that can still make you sick. Same with the antivirus, there are new viruses for the computer released every day and there's times when your antivirus provider can take a week or two to write a detection pattern for that virus.
5 - Update your virus patterns daily. As I said before, new viruses are released everyday.
6 - Don't be affraid to cross check your computer with competing antivirus vendors. Many have free online virus scanners (such as trend http://housecall.antivirus.com
7 - Know your source. Software you've downloaded off of kazaa, imesh, edonkey and any of the other sites is worth what you paid for it and is often a source for viruses, trojans, worms and other malware.
8 - Consider switching to a non-windows operating system. Linux, Unix and MacOS have fewer viruses combined then you can count on your fingers and toes. Macs are known for being easy to use and now with they're latest release have the stability of unix. Windows also copied Mac on it's interface and mouse and has been playing catchup in usability for years. Heck, there's only one mouse button so there's no remembering to right click or left click or stand on your head and wave a pinapple while trying to navigate through menus. If a Mac is too complicated for you then maybe you should switch to "WebTv" You can't get viruses on that either. Linux and Unix aren't that hard from a user stand point and are getting easier every day. The operating system is free and there are support packages available out there to give you the same technical support you can get with certain name brand PCs for windows. There may not be as many games, but there are games out there for all platforms.
I could go on but I think that's more then enough for now.