[Moonliner]

Based on your description, It does not sound like a heat problem.

With a stuck fan or dusted up cooler you would still be able to boot but the PC would hang/crash after some random amount of time. A problem serious enough to kick in during boot (ie no heat-sink at all on the CPU) would quickly fry that component resulting in a very solid error.

It sounds like you have (excuse the technical term) fuddied up one of the core Windows drivers. When/if you find the XP install cd you should be able to run a repair. I don't suppose you have an Rdisk handy by any chance? Nope. I thought not. No one ever does.

Note: This does NOT imply you should not follow Matts advice. It's possible you fuddied your PC with a heat related issue as he described although I would guess it was the software you installed.

Also be sure to re-run the windows update program after you run the repair (assuming it works).