Windows XP?
Create a second NTFS partition called "D" and move your files you want to keep from C to D.
Then do a quick format and reinstall XP on C.
That's what I do several times a year and I've never lost any files. I still backup stuff to CD though as time permits.
After your reinstall, use Spybot, Adaware, Firefox, and a software firewall (I recommend Sygate). Be careful what you download and open. Don't use IE or Outlook. Use web based email
I never get spyware or viruses.