http://onecare.live.com
It's a MS online scanner that will do virus, spyware, registery, disc issues. I've used it twice, third time counting the current running on another machine. It's for XP, the first time I used it I had been pulling my hair out with my new laptop and it was down to 'run it' or 'hit the blue IBM recovery button' that would have reimaged it from the image stored on the rescue partition on the disc. That would have entailed a good 8 hours of reinstalling my CVS repository and a bunch of other building software that I had just spent the past 3 days installing and getting to run. Both times it has saved my ass. I'll also be running it again later on on another machine that I have had recurring issues with, hoping that resolves those issues.
Run the full system scan, be advised it will take a while and you need to use Internet explorer, it won't do the directx with firefox.