If you're using Win 2k or above, turn off the auto restart on crash (Control Panel->System->Advanced somewhere). At least then you'll get blue screens instead of reboots.
If you have fast boot enable in the bios, turn that off - bad memory can cause crazy things to happen, and the slow boot should check memory (a guy at work had a bad dimm 411Mb into memory, which caused random blue screens).
Unplug everything, remove each device (card, memory, processor, etc) and reseat.
If it still acts up after all that, start throttling back bus speeds, disabling processor caches, removing/replacing components till you remove/replace the bad bit.