Check how much of your hard drive is free. XP will slow down if less than approx. 20% is free.
Check via Start->My Computer->right-click on drive->Properties. See what Disk Cleanup suggests.
Do you regularly defragment the hard drive? Files are routinely chopped up and spread out on the disk(s), whenever the PC is in use. This slows things down. XP has its in-built defragmenter, but the free one from Auslogics is faster. Details at:
Disk Defrag - Reclaim the Speed Of Your Disks For optimum results, I would suggest that you boot into Safe Mode, then defragment. Windows accesses less files in Safe Mode, so more are available to the defragmenter.