Seriously, I need someone to help me out!

I've had this problem for a long time now. Whenever I play a game, could be Age of empire or Counter strike ... or M2TW , My PC will reboot.. without giving me any errors or bluescreen.(Must be hardware problem)
This past month It havent happened, I've been playing Company of heroes, oblivion, CS:S without any reboots at all.
But for 2 days ago It happned again!!!

And I dont know how everything began, it just one day started to reboot. I got myself a new PSU... same problem... cooler and fan to the GPU .... same problem. And I have a case fan! I also got myself a new CPU Fan, but still the same problem. What could it be???

I have also always updated drivers on everything.(But havent been able to even find the driver for the motherboard)

I've got a Fujitsu siemens scaleo 800 :
Pentium 4 3,4GHz HT (Its a prescott.. known for overheating)
1024 Ram
Ati radeon x800xt 256mb
... tell me if u need more specs on anything.

And I've got XP Pro Service pack 2

Thanks