I know the psu works because the fans and dvd drive still turned on.
I hear what your saying, but unless you have the tools to measure all the voltages, including the AC component, try swapping the PSU. Borrow one if you have to. Try this before you spend money on a new board.
As someone else pointed out there are multiple rails. One out of spec rail can keep a machine from booting, but simce the primary rail is the 12V rail, if it's ok the fans will run and the drives may spin up. My current machine became unstable and I found there was excessive AC on all the rails and the voltages were borderline low. Replacing the PSU solved the problem and its been rock solid since.