You could try going to add and remove programs and un-installing everything apart from nvidia/ati drivers,
media player,
windows internet explorer,installer 3
audio drivers
and microsoft office/updates first,
Then do a defrag
Then un-install anything else you dont want, left in folders i.e. remanets left of files, games,drivers etc,
Do another defrag
download and install ccleaner or download it now and save onto disc, and run to clean up the registry,
http://www.ccleaner.com/
DO NOT UNINSTALL ANY DRIVERS TO DO WITH YOUR O.S
hope this helps
good luck