In addition to AVG and Ad-aware as freebies, get Spybot Search and Destroy. It is so good, it is mandatory if you really want a good cleanup.
If still problems (IE lots of porno use) , try a registry fixer like Registry Mechanic but that is a last resort really. Try going to nightcubs at 4:00am with $50 for drinks, it works better than having to 'clean' your computer so thoroughly
