Crashing games and error message during post. plz help!!!

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Would anyone know what could cause my games to crash and close down at varoius times while i'm playing? Sometimes it won't happen for hours while im playing while at other times the game will crash after a few minutes. This happens in all my games, then the ususal message appears saying "error this application closed down....blah blah...sorry for the inconvinience....send...dont send. Also during post i get a message saying <bios> inccorect bios update..... I got an Asus p4s800 with the latest 1009 bios. i tried going back to my original 1006 bois but i get the same thing