I'm always suprised how much people are afraid of viruses and shit on their PC's. I guess the difference is that it takes a little diligence vs none...
It is incredibly easy to secure your pc with free software that has a very small footprint, I've been using avast! lately as the Norton that comes free from my cable company seems to be constantly locked up/broken.
If you keep your data in order, reinstalling windows7 from a thumb drive is a snap too. I tend to do a window wipe every 6-10 months whether I need it or not. Nothing like a clean registry and naked partition full of fresh installs.
Seriously man, I run Microsoft Security Essentials as my AV, and Malwarebytes as my virus hunter/killer.
I schedule them to scan once a week, and I haven't had a virus problem in 3+ years. That's with watching a respectable (normal) amount of porn.