I recently attempted a repair of a co-workers laptop. I tried some of the free stuff available. I eventually installed Norton Internet Security 2009 and did a scan. Excluding tracking cookies it had 681 viruses, trojans, worms etc. His browser showed hundreds of porn sites.....but he told me he doesn't go to porn sites. You should have seen his face when I showed him his browser history!
Anyhow, there was so much damage I ended up doing a format and reinstall. Works every time.
I always say "sleep with the dogs you get the fleas"