XP freezes solid............................

Usually MB, memory and Power supply errors give a blue screen and then they reboot...USUALLY. HD errors on the other hand freeze up cold, usually with a blue screen. In my experience they don't always reboot...but either freeze up or bluescreen freeze up.

Have you checked the eventviewer? What does it tell you?

It might also be a virus or spyware. Try in safe mode, run a complete antivirus scan, spyware removal scan with multiple spyware scanners (Lavasoftusa.com, Hijackthis, and Spybot) and see if you find anything.

Does it do it if you boot it in Safe Mode?

If it behaves in Safe Mode, boot it normally, run MSCONFIG, start disabling things ONE AT A TIME, writing down what you disabled, then rebooting after each one. Start with the Startup tab; leave the Services and Boot tabs alone. If that doesn't help you find the problem, go to Hardware Manager and start disabling PERIPHERAL devices ONE AT A TIME, writing down what you disabled.

Check your processor. If it's getting hot it will lock your system up. After your first lock up and you restart does it lock up faster the next time? If so it's a good sign that your processor is getting hot and quitting.
A power supply can do the same thing also.
If you want to test the memory you can download Microsoft memory test tool, do a search for it.