Hi I just bought a new motherboard and video card since my older setup was doing the same thing. I just got a new HDD too so it shouldn't be it either.
Ok, my build is
Socket 939:AMD athlon 64 X2 4200+ Toledo with Zalman fan
motherboard: asus a8v-xe
ram: two Corsair XMS ddr400 1GB
video card: ATI Radeon HD 3650 PCIE
HDD: Seagate LP(lowpower) 1TB
PSU: Antec Neopower 500W.
I'm pretty sure it's the power supply. I had it for about a year now though. At first I thought it was the PSU, but then I started to suspect it might be the memory, I did memorytest86 and computer restarted same way as playing games. But I replaced the ram with another (same brand ddr400 different size: two 512MB ram) and I get the same thing while gaming.
Help me figure out what is the true problem on my pc so I can get a warranty replacement for either the PSU or RAM (Yeah my build is old, that might be a problem to some of you