My computer keeps on shutting down a few minutes after startup

every time i startup my computer, it starts up just fine then after a few minutes, probably around 10, it shutsdown. My heatsync thing or whatever is fine now i just put some new stuff on it and i thought that would solve the problem, but it just made it start up faster   but it didn't fix my problem. Can someone please help me? Thanks. oh yeah and it sends error messages when it starts up and the windows website says my bios needs an update and my nvidia 6600 geforce does too. My Motherboard type is ... Phoenix techs LTD 6.00 PG, 8/22/2002