Computer restarts. Please help!

Here is the pattern:

First, I was able to start windows and launch some progrs. But as soon as i would try launching a game, media player, or other stuff the computer would reboot...

I logged in Safe Mode, and everything seemed to work. But starting Windows normally would cause the same thing. I figured it could be a virus or a problem with video card drivers...

Virus check did not show anything neither did reinstalling the drivers. I decided to do "repair windows". During the "Installing devices" stage of installation computer reboots. Now I am almost certain its a problem with hardware.

Please help me out... any input appreciated.

I have a self-build rig:
Motherboard: CHAINTECH VNF3-250
CPU: AMD Athlon 64 2800+ (with stock sink)
Video: Radeon 9800PRO 128
RAM: CORSAIR XMS 512MB PC3200
PSU: Antec 430 True Power

I have updated the bios version to 27, I believe. The system was not overclocked in any way when the problem have started. I will gladly provide any other system specs upon request.

Would it reboot fine or would it freeze? Sounds like either the mobo or PSU is going out if it will reboot during an install/repair. Try reformating and reinstalling Windows and see if that will clear up the problem. Otherwise it is a hardware problem in which you will need to start isolating the occurences.

Thank you for advice.

The machine does boot up without problems. It only restarts during the instlation process.

From my understanding, if the machine starts it means that the CPU or/and RAM are ok ? The fact that it doesnt reboot randomly also supports this.
At this point what ahrdawre does it leave in question ? Could the video card cause the reboot?
And is there other ways to isolate the problem, other then replacing the components at this point ?