Ok, update since yesterday -

Here's what all I've done now -

Dug a cd copy of SP2 out of my (badly organized) cd case, and installed it (after letting the computer have an hour or 2 of cooldown time). Success!

Went to the parts store and got a new cooling fan for the cpu, and one for the front vent on the box - we now have wind tunnel effect across both the ram and the cpu and they're cooling quite nicely.

Spent the morning updating windows everything so now I'm fully up to date on all my M$ software, as well as windows.

Result:

For some weird reason, installing sp2 seemed to fix the problem somewhat - I went and bought the fans because I still suspected overheat, and sure enough, no more bluescreens to right now - the computer has been up and running since 8:00 this morning, it's now 2:30pm, no overheating, no crashes!

Thanks to all of you for the advice.