New ASUS A8N SLI not booting. Power supply or motherboard?

[bail]

Installed the a new ASUS A8N SLI motherboard into the case and switched on the mains. All the fans started straight away (didn’t press the power on switch at the front). However, the motherboard didn’t give the a power on beep or start the monitor. I was not able to switch off the computer by pressing the power on/off button on the front, I had to switch it off at the mains. Thinking it was probably a short I took the motherboard out the case and tried to get it work on an insulated surface. Still the same. Tried removing the components one by one (sound card, gfx card, memory, cpu) still the same. Even with no components in the motherboard I should still get a POST message telling that I have no components installed! But, nothing, no beep at all. Also, it still has the same issue were as soon as it gets power all the fans start (front power button not pressed) , and pressing and holding the front power on button does not stop the fans. In fact I can unplug the power on connector from the motherboard and it will start the fans just by switching on the mains.
Any ideas? Could it be the power supply (its about 2 years old 450W, not sure of the make)? Thanks in advance for the help.

[PC TECH250]

If the supply displayed no problems with other boards which is the immediate reaction here
you have one of two possible causes. One would be the less likely mismatch of wiring which
can often be a problem. The second is a board defect where the supply circuit on the board
itself is "foobar". When first turning on a power supply breaker NOTHING should run. All fans
and installed hardwares start running when the power switch is used. First perform a total
review of all connections to the board along with no reversed connectors before assuming a
faulty board has been received. A bad supply wouldn't see fans run before the power switch
was first used on the front of the case. Reversed plug(not likely), miswiring, or board defect.