If you want to get your computer running in tip top shape I suggest you read Koroush Ghazi's Tweaking Companion. You'll find it on
www.tweakguides.com. It's a 2Mb or so Adobe file that you can download for free and will teach you everything you wanted to know about Windows XP but were too embarrased to ask.
I still use it on a regular basis and cannot recommend it highly enough. All you want to know about cleaning and optimising and then a dash extra. Also, he actually recommends Freeware utilities to optimise your system and directs you to where they can be found. (All from reputable sources such as MajorGeeks and the like.)
Let us know if you find it helpful.