Sudden Hard Reboot

I need some advice. My computer (emachine, AMD3200, 64Bit, 1024ram [2x512], DVD-RW) keeps going into a hard reboot. It usually happens when using a video or flash related program but not restricted to these. I can be we surfing and it will crash. I can also no longer burn DVDs. When I get back to windows I get the "your system recovered from a serious error...blah, blah, blah. I sent the error log to M$ and get a web page telling me it was too vague and it's either RAM, CPU, Motherboard, or Power Supply. I've DL'ed some test programs but the machine crashes before finishing any of them. I tried pulling each ram bank separately and run a ram checker and it found no errors on either. But I can't run an "over-all" without the hard reboot. Any one have any ideas? I'm ready to junk the stupid machine but can't afford to replace it right now.

 

I had the case open yesterday and it look relatively clean. I used memtest. I tried running Hot CPU Test Pro but it crashes before finishing. Do you have a better Freeware suggestion?