BleepingComputer.com: Computer Locks up/Freezes

The PSU is plenty good enough for that Graphics Card (requirement is 400W).

If you have tested RAM, Graphics Card, HDD; it could possibly be your CPU. I would say that it definately isn't your Motherboard. I also don't think that heat is your problem. So, try testing your CPU firstly, if that isn't the problem, try disabling/taking out your Graphics Card and running the computer with only onboard graphics. If this fixes it, Tada! Graphics Card problem. If it doesn't, well..... It's either gotta be RAM, CPU or HDD.

Are you positive your OS install isn't flawed somewhere? Try chkdsk. Is your HDD full or close to it? This could be a problem. Try a memory check, i'll link a download for one if you need.

Ummmm Defrag?

Also, out of curiosity is your XP x64?

its not all about the wattage though, it also has to do with the amerage. the minimum PSU for my 3870 x2 was 500 watts, and I had a 600 watt PSU, what I didnt know was that the 3870 x2 needed something like 26 amps or something similar minimum on the 12v rails, and that 600 watt PSU only had like 19. soooo the video card overloaded that 600 watt PSU even though it had 100 watts more then it needed, until the PSU finally fried. that is why I replaced it with a 750 watt PSU with 4 12v rails each running 19a each, and was crossfire enabled. Now I dont know the exact model of your PSU, but I would definetly check the amperage of your 12v rails and see what that says. And if its NOT your PSU, then its definetly, a cooling issue with the video card. either the cards not getting enough cool air to cool it down, or the cards defective. Like I stated previously that card SHOULD NOT idle that hot, for any reason whatsoever.