Sounds like a 'software' problem to me, XP is still not perfect, and is often affected by some of the tweaking things we do.
Several suggestions. Run a good 'registry cleaning' program to get rid of the junk. Run a simple performance program to check out different pieces of hardware, such as Nortons "Performance" or one of the freebies off the internet. This will either identify the hardware that is not performing or will tell you that the hardware is OK.
A reinstall of XP will sometimes clear up software problems; and a reformat clean install does wonders. Time consuming, etc. but as a last resort, it usually works.
If doing the above does not work, I would suspect the hard drive, as every now and then we have hard drives that slowly fail as well.
But, before I rebuilt my computer, I would resolve all software issues first.