Most common causes of freezing is RAM and/or over heating. Commonly however with RAM it just won't boot - though can cause freezing if it's on the way out.

1) Clean out the system with some compressed air. Aiming specifically for all your fan's (Case, CPU, Graphics Card, PSU - Dont open it but, just aim through the vents).
2) Strip the system to the bare necessities, make sure the processor is seated firmly, and the fan is spinning freely on it. Unplug all drives (Hard disk, floppy, cd/dvd), take out any extra cards such as USB/Firewire expansion cards, sound cards etc.
3) Start the system and trial it for a period of time. You may also want to try swapping out the RAM (if you have multiple sticks - IE 2 x 512MB). If it still freezes, best bet would be to swap the RAM with some you know works - out of another system, or you can pick up RAM cheap as these days. If it run's fine, then start adding items back in one by one, keep it running for a period in between each one until it starts freezing again.
4) If it starts freezing again, take the parts out one by one in the reverse order that you put them back in until it stops. You should then have some idea of what is causing the issue.

Being that you have re-installed XP and replaced the hard disk, chances are pretty high that it's not a software or driver issue. You could check with the manufacturer of the offending item to see if there is a firmware update available.

One other option would be to try your RAM in another computer, see if it causes the freezing issue, then run Memtest over it to see if it finds errors.

Feel free to PM me with any questions

- Troy

__________________
Osama Bin Hooning
Senior Terrorist Operative
Perth-Wrx