I suggest you buy the excellent defrag program Diskeeper in order to keep your Windows system operating at full blast. Apparently, according to my Diskeeper box, you can increase your performance by 200% just from using it. I also advise the purchase of a fully automated virus protection program. Symantec and McAfee both have excellent offerings in the way of virus utilities.
If you can't afford Diskeeper, run the built-in defrag at least twice a week. If you can't afford virus protection software, you have my condolences.