I have just tried to turn my PC on this morning and it won't stay powered up. It will come on as normal for about 5 seconds, not quite long enough for the monitor to wake up. Then it will power off. After about 2 seconds it will start again, it will continue like this until I pull out the plug.

Plugging it in again doesn't restart the cycle although pressing the power button does. I have tried to clear the cmos but beyond that I have no ideas. It was working perfectly when I put it in hibernate last night.