Repeated Motherboard failure?

I'm working on my son's machine right now - it failed before Christmas with a popped psu which took out the motherboard also. Symptoms were - a tripped mcb on the house consumer unit and a machine which wouldn't boot/show any signs of life. When I took it apart I tested it with a good psu and found signs of life (cpu fan and hard drive started) but no inclination to boot. I tested the component cards in another machine - all good. Bought another psu (Zalman 500w) and a motherboard (Gigabyte) - had to re-install XP to stop it bluescreening, but so far so good, until it simply died again for no apparent reason. This time it is completely dead, no beeps, no sign of life whatsoever (except for the mouse diode) with a good psu - I suspect the Zalman is still OK and the board is faulty or blown.
So now I'm scratching my head - have I just been unlucky to have a faulty board, or is something else causing this problem? What could cause this? Might there be an electrical issue with the case? I have a Scythe Kama cooler on the CPU - it seems to be OK but could a CPU overheat or fault do this?

Grateful for any advice - not sure what to do next. Don't want to blow another board.

Reuben