If following Buenchico's advice doesn't help much, there are free guides to improving performance on XP and Vista at
http://www.tweakguides.com/TGTC.html
Be warned they are rather long and can be a bit wordy because they are written everyone from novices on up, which means there are a lot of explanations in there. Ignore any sections which deal with cleaning up or editing the registry, not because the advice is bad, but because messing with the registry really isn't a good idea.