Dell Dimension 5000; PSU or motherboard problem

Hi all,
My mum was doing some accounts on our Dell Dimension 5000 earlier today, when it suddenly just shut down. When attempting to restart it, the power button instantly flashes amber continuously, but nothing else happens; no diagnostic LEDs, doesn't POST and the fans don't spin up. Upon opening the case the AUX_PWR LED on the motherboard is continuously lit. I'm guessing that either the motherboard or PSU have gone, but need to narrow it down; in the case of the former it'll be a total rebuild, in the latter case I'll just replace the PSU. What do you think?

Sorry for the bump. I've just tried testing the PSU by shorting out the green and black wires on the 24-pin plug, but nothing happened (Ie fan didn't spin up). I then tested the voltage across the wires and got 5v, so can I assume that the thing is dead?

I had the exact same problem, I just replaced the PSU for a 24 pin ATX with SATA, I bought it from Misco The exact model was a Taurus X P430w ATX 12v ver 1.3 And ATX2.03 compliant. Ultra Quiet Technology.
Hope This Helps.
RealGoneKid