In the 20+ years using MS products I have gotten two viruses/backdoor programs. One was from a Wolfenstein 3D disk I got from a buddy which didn't do much harm to a Win3.1 system and a backdoor program I got from downloading a dodgy screen saver in Win98. Not going to dodgy sites (such as Warez) and downloading/using suspect software helps alot. If you are loading software to help you cheat, steal passwords, hack a system, or otherwise do something malicous, expect a virus. That seems to be the way it works.
Spyware, on the other hand is a real nuissance. I abandoned Internet Explorer because of all the holes and exploits that allowed spyware. I now use Firefox and MS anit-spyware (beta) which seems to have solved all those problems. Currently I used Norton Anti-Virus, which is very intrusive and has never found anything other than a suspect program through email (that I would never have opened anyway).
For me, I am going to go with AVG anti-virus free edition because its free and can be configured for minimal intrusion. I *have* had Win98 systems corrupted because of Norton system works! Right now, two of my windows PC's are running "unprotected" and I don't expect any problems.
While I keep messing with Windows XP, I really gotta go with Debian or FreeBSD in the future. It would be cool to see something like OSX come out for PC's too. Anyway, AVG is what I plan to use in the future.
Regards,
Malta