there are far too many words on this thread for me to see them all.
if your computer is overheating, then there are 3 places it would do so... no, 4.
1) the processor - if you have a fan on there and it isn't overclocked, then that is not likely it (esp if it worked just dandy for some time)
2) the ram - this is only likely it if you have had the computer some time and there is dust on the ram
3) the video card - this is highly likely b/c xp has different (better) drivers for many of the newer cards for the newest directX BUT they tend to push the card to its upper limits and they crash a lot - this was a big problem with the Nvidia GeForce cards (esp if you have the VIA chipset on your motherboard).
4) harddrives - as your disk gets more fragmented, it has to work harder to search the disk to find shit, and more work means more heat.
you can tell if it is an overheating problem by the amount of time it takes for it to crash.
put your system under full load (distributed.net will help) as soon as it starts and wait to see what causes it to crash and how long it takes.
then have it under no load and see how long it takes.
the higher the load, the higher the heat - and then longer time also means more heat.
you could also take the case off and have a housefan blow on it, that was what I did much throughout summers in college b/c they didn't have AC there.
but personally, it doesn't sound like overheating to me.
it sounds more like the driver you have and xp aren't playing nicely together.
since xp is relatively new, some driver people aren't bothering to make the newest drivers for things - or in your case, perhaps the newer driver is out there, but you don't have it installed.
have fun