Motherboard problem? PC not booting (no signs of turning on).

The PC that I built a few years ago suddenly will not turn on. I've had it on constantly for a long time. Come home yesterday and it's off. I press the switch, and it momentarily starts to power on (like 2 seconds) then dies. Does this three times, then nothing. No signs of starting, no fans or anything.

• I've tested the PSU (FSP Group AX-450PN) by disconnecting it from the MB (Asus P5B) and shorting the Power On (green wire) to ground and it runs fine.

• Green LED on MB is on when PSU is switched on.

• When I switch the PSU off, there is a ~1 second delay then fans spin for about a half second.

No expert but I'm guessing MB issue. I've read broken case power switch can cause similar problems, though doesn't seem likely. What further tests can I perform to identify the problem source?