Computer dead, but fans whirr away!

OK, I turned the computer on this morning. The LED lights,the fans whirr away, but no bleep and no picture on the monitor. The fans seem to be speeding up then slowing down. I can't turn the fans off either with the front power switch, I have to turn the power supply off from the back. I took the lid off and it seems the power supply fan and CPU fan are working (the CPU's only spinning slowly though). No hard drive response at all.

The computers an ex-work jobbie, about 2 yrs old now, fitted with a Sempron processor (I think 2400+ not that I really care, it's just a web browser), crappy Asrock mobo and 256Mb ram.

I'm figuring either the power supplies had it or the mobo's died.

Can anyone tell me without me having to wire up the power supply from my other computer to find out?

EDIT
Actually, I've just remembered there were lots of mini power cuts going on yesterday. The computer would have been on all the time but it was working fine when I got home. Could this be due to a power surge?

EDIT EDIT
OK, now it's totally dead, no lights, no fans, no action :(

Sorry for the late update, it was the power supply that had gone phut.