Almost certainly the power supply - unless there's been a catastrophic failure elsewhere that's pulling the 12v output down.

Quickest, easiest and cheapest (I like cheapest!) way to test is to take the side off the PC and put it alongside a PC with a similar PSU and then plug the good power supply into the poorly PC.

Or buy one of the rather fancy PC power supply testers (Bernard's got one and it's brilliant!) and connect the power supply to that to test it. They're not at all expensive here in UK - around £20 to £25, and they're a very useful piece of kit to have in your armoury.

John