All Games crashes

Hi!
I have bought a new PC.
CPU - Athlon 64 3500+ S.AM2
MB - NForce 550 S.Am2 Biostar
VGA - Gf7600Gt PCI-E16 256 Mb Giga-Byte
RAM - Dimm 2x512MB PC6400 DDR2 800 MHz A-Data
PSU - Atrix 500W
SB - Creative SB live 5.1 digital
When i installed games on my PC i realised that all games crashes after 5-30 min.
Temperatures are Ok (Cpu 23, VGA 40-60, MB 40, Hdd 40 celsius)
PSU voltage are normal
I installed all the newest drivers, but games still crashes. I don't know what can I do to fix my PC. Please someone help me!
Thanks!