Dead mobo or something else?

[stopstartstall]

So I've just put together my new build. It ran fine for a couple of days then it died this morning and won't boot up. Specs are:

i5 760
gigabyte p55m-ud2
4gb corsair xms3 1600mhz

I suspected the power supply (it's a few years old and I was worried about it being underpowered as it's only 400w) and borrowed a psu from a friend which works. Computer still wouldn't boot, but when I press the power button, the cpu fan "twitches" and the leds on the mobo flash very quickly.

There's no shorts anywhere between the case and the mobo, I've unplugged HDs and DVD drive so there's the minimum of stuff plugged in. The only thing I haven't tried is swapping the ram or trying one stick cuz I'll have to remove the heatsink and that's a pita and wanted to ask here first, lol.

So is it likely to be a dead mobo, the ram or the cpu? I'm guessing it's RMA time, I wanna know what it's most likely to be.

 

[RJC]

Was the system overclocked, try a CMOS reset.

The other option is to remove the memory just in case it is a faulty / problem module.

 

[stopstartstall]

I've tried resetting CMOS. I'll try the ram, trouble is I don't have any other ddr3 to try so the best I can do is try each stick in different slots.