I'm really not sure what it is. If the CPU was overheating, I'd expect it to collapse after a few minutes, max. If you know what you're doing, try downloading and running Knoppix:
http://www.knoppix.org/
It's an operating system like Windows, that you download and burn to a CD. Then when you put the CD into your computer and restart from Windows, it loads Knoppix. Can be very useful sometimes. It won't be using any of the software or drivers that come with Windows, so if you can run Knoppix fine for much longer than you can run Windows before it crashes, then it sounds very much like some software problem in Windows.
You just have to make sure that in your BIOS, its set to first try and boot from the CDROM, then from your hard drive (perhaps IDE-0 in BIOS). That way it'll load Knoppix before it even tries to find Windows. Then you can restart Knoppix, take out the CD, and Windows will run as normal like it always has, without any knowledge of you running Knoppix!