I can't find computer problem!

[LH-Danny]

I currently have a computer which has quite a bit of a problem that I cannot locate. Whenever you try to boot up the computer, it passes the BIOS, and power on test but once it starts to boot into Windows 7 the computer automatically switches off.

I have tried to locate the problem, and have tried the following things to fix the problem;

Reseated all cables, still wouldn't boot
Removed the hard drive and put it into another computer, and it successfully loaded into Windows.
Ran a RAM test, no errors
Ran a hard drive test, no errors
Used a different graphics card, still wouldn't boot
Used a different power supply, still wouldn't boot
Used a different CPU, still wouldn't boot
Took a hard drive out of another computer and put it into the one that wouldn't boot & it still wouldn't
Replace the RAM, still wouldn't boot
Removed all PCI card, still wouldn't boot
Disconnected optical drive, still wouldn't boot
Disconnected all peripherals, still wouldn't boot

I have replaced or disconnected almost every component within the computer, apart from the motherboard and once it got to the Windows load screen it still shut off. The only component I have not changed is the motherboard, so it must be at fault?

Thanks for all help!