seems like a power supply issue to me but i'm not sure. My mom's compaq used to do the same thing. It would start windows and then when it randomly restarts, it doesn't load windows again. Just like you, the fan spun but nothing happened, no screen, no window logo, nothing. All i could trace it was to the psu, but unforteunately, i didn't have to fix it. We just bought a new compaq because it was already like 5 years old. I think it's the PSU, try using a different powersource and see what you get.
edit: oops, you already tried a different power source.
this is wacked... Let me try to get some information from other places. bbl