Friend bought his computer to me because windows wasn't loading and would just keep restarting.

i took out the HDD backed everything up and started a fresh windows install.

halfway through it just powered off and refused to switch on. I get a quick flash on the keyboard LED's, and the fan moves half a cm.
I tried a different PSU, RAM and CPU all to no effect.

Apparently this all started after a power cut in his area, so i guessed a cocked up PSU and motherboard and ordered a new one of both. exact same gigabyte motherboard and a new corsair 400W PSU (its a low end PC).

Switch it on with old CPU, memory and HDD's etc and it all works fine. load up windows install and in the exact same place it just powers off and now refuses to switch on.

Faulty HDD, DVD's are unlikely to cause a power faulire on the motherboard as far as i'm aware, any other clues about whats causing this?