No splash, PSU fan turns but stops

I've been setting-up an old computer for use as a media centre in my living-room and I'm having some problems. I installed windows MCE and got it working, then decided to try-out Linux (Mythbuntu). When I installed Linux I accidentally overwrote the boot sector of my windows install (my own fault, didn't use the default settings). Still, I used Myth TV for a bit, and it worked OK, but the drivers for my card were dodgy so I decided to try and get windows working again.

I put my windows disk in to repair the install and, as the CD was loading, my screen went blank and the computer wouldn't respond to any inputs (ctrl-alt-del etc). What it did finally respond to was holding down the power button for 4 seconds.

When I tried turning the computer back on, nothing happened. There was a little noise, but not much. Opening the case, I identified the noise was the PSU fan beginning to turn but stopping. The power light on the motherboard (an A7N8X-E Deluxe) does turn-on when the PSU is on, but nothing else seems to come to life when the computer is plugged-in and turned-on.

What do people think is the problem? I've had this problem before with a different computer, the problem then was a short between the MB and the case. I don't think that's the problem this time, because the computer had been running for weeks since the last time I tampered with the hardware in a way that might cause a short.

I'm thinking it might be the PSU, but would like other opinions before I go out and fork for a new PSU.

The LED on the motherboard is powered by the 5 volt Standy PSU output. This is a small 5 volt power supply that is separate from the main part of the PSU. As you have noticed, it is always on as long as the PSU is plugged into AC power and the hardware power switch (if present) on the back of the PSU is on. This does not mean that the main outputs of the PSU are good.

One thing you can do, if you have a second computer, is to pull its PSU and test by substitution.