That is a pretty vague question. Do you delete temporary internet files, history, disc cleanup, etc. on a regular basis? Do you check for viruses, spyware/adware/etc. on a regular basis? Do you empty the reycle bin after using it?
More than 60 people whom i have asked have experienced that windows XP works slow after about 25-30 days , why is that????????

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic