Computer shuts down 3 seconds after starting up.

Recently my computer started shutting down on its own. Eventually it would start for a couple seconds and shutdown before windows even boots up.

I thought this had to be the power supply, so i ordered a new one and replaced the old one. the new one booted up to windows... and then shut down on its own, now it's shutting down a couple seconds after I start it up just like the old one.

If i dont try starting it for an hour or so, it will boot up all the way to windows, but will shut down by itself, and then it will go back to shutting down a couple seconds after starting.

I don't have a clue what this could be... I was SURE it was gunna be a power supply problem, but that doesnt seem to be the case now that it's still going on with the new one. Anyone with any information PLEASE help!

this is off the top of my head as i am at my girlfriends house...

evga nforce 680i mb i beleive
2 gig ocx ram
evga gforce 7900gs
500 gig hd
2.2ghz dual core intel
the old powersupply was 600watt ocx new one is 550 coolmaster

if the specific specs are more important i will reply again with the exact information!

sorry for such a newb question but what exactly does HSF stand for?

same thing was happening to me on an am2 system, if reseating the hsf dosent work, try getting a new cpu, a cheap celeron or something