Thanks for your quick reply,
Is it possible that a PSU can half-work? There is power coming from it otherwise the mouse won't light up through the mobo. Could it be that only certain rails are working?
I hope you're right though, I'd rather it be the PSU than anything else!
Yea it is. I had a psu that light up my mobo light and my case led, but the computer would not boot. I got a replacement and that worked like a charm. If that does not work, then try switching the panel connections or make sure your mobo jumpers are in the right place.