Here is one of a handful of legitimate "clean up" utilities that are safe. Read help file, 'tho and make sure you aren't getting rid of critical cookies, etc.
http://www.stevengould.org/software/cleanup/


I use this with XP Pro and it does help a lot --
Do you use "Adware" or something similar in addition to a virus checker? If not, might consider this -- "adware" and "spyware" can REALLY choke up your machine.