both westin and mlf are correct.
i find that even if i try to clean things up, i need to do a reinstall every 6 months or so. it's suspect its because i install and eval alot of software. Even if I uninstall, it still seems to bog down. If this is the case with you, i would suggest you take westin's advice and reinstall.
If you have run the usual cleaners and av checks, you might try to defrag your disk. you also might check to see what services are running and disable things you dont need. There are some decent links on what services you can do without. e.g. http://beemerworld.com/tips/servicesxp.htm
For a sure cure, install a small linux dist. ;)
CSR

