A major cause of slowness is overheating. Open the case and make sure there are no accumulated dustballs or other obstructions to fan operation or air circulation. You can monitor your temperatures with (free) Motherboard Monitor.
Not enough RAM
If you have less than 512 MB, consider adding more memory.
Defragment your hard disk every so often. The defrag that comes with Win XP is fine, or you may prefer O&O Defrag 2000 Freeware which has more interesting graphics. For defragmenting your pagefile and registry hives, I recommend (free) PageDefrag.
Some legitimate programs are known to use a lot of resources and slow you down
Those described in replies below do not need to be running. If you think that is your problem, please post a HijackThis log and we will help you keep them from starting automatically.
Edited by cnm, 16 December 2005 - 10:57 AM.