Start with formatting the hard disk and installing all software (including OS) again. Did the system come with a system restore CD? Then it's easier then if it o(only) came with a Windows CD.
Make a note of all your settings, and a backup of everything you don't want to lose. And don't forget to visit Windows update afterwards to re-apply all security patches.
Your machine will be as fast as it was after such an operation.
This is a kind of random question, but does anyone know any programs that like, scan your system and find all the hardware updates you need? My computer (2.1 ghz, ATI Radeon 9600, 512 MB RAM) is running REALLY slowly, especially on games which used to be smooth as can be. I've updated the games, defragged my system, I have no spyware or ads, I have checked 3 or 4 virus checkers so I think i have no virusus, so my only guess now is to try and update all my stuff. I dont know how much it will wokr but I have to try because it's really bothering me since I just recently purchased this computer and dont have money to upgrade or anything. Thanks for any help or additional tips/