It must be you. I'm on my third computer in 8 years. My first lasted 3 years and became unusable due to various components locking up, most likely due to viruses. Second went 5 years and has never had a virus. My third is only a month old and is also virus free, unless you want to count Windows Vista.
Damn thing is ALWAYS whirring, downloading or computing. When I fire it up, it's a solid 5 minutes before the damn thing quiets down so I can surf at normal speeds. I'm almost ready to shitcan it and go back to my old computer with XP.
Sorry for the sidetrack. What firewall are you running? Make sure it is up to date and run a full scan. Then set it to run a full scan at least once per week. You can schedule it to run at 3am or something when you're not using it.
Go to www.lavasoft.com
and download the free version of Ad-Aware. Update it and then run a full scan.
If you're running different user profiles on your computer then you may need to scan each profile, I'm not exactly sure how that works.