Windows 7: Computer froze and now doesn't even boot.

I was busy copying the latest Diggnation episode to my iPod with DiskAid and some music on in the background (with WMP). Suddenly, everything on the screen freezes, but the music keeps playing until the end of the track. Rather concerned, I tried to start task manager, nothing happened. I held down the power button, didn't switch off. So I unplug the power cable, plug it back in and the PSU fan and heatsink fan start up (I hadn't pressed the power button), very loudly but nothing else happens. No visual output at all. Then the BIOS beeps (after about 20 seconds of fans) 'beeee beep beep' and then the fans just carry on. I unplug it again, then remove the HDDs and the DVD drive. Try again, nothing but fans and the same beep. I then remove the graphics card and plug it in. Same, but no beeps. I then switch the graphics card, replace the BIOS battery and plug it in. The fans don't automatically start this time, but when I press the power button, they do. I then remove the RAM. Same result again.
Any ideas on what is wrong with it? This is my primary computer and the next best computer I have is an immovable HP ZV5000 laptop with just 512MB RAM and a dodgy HDD, so it is pretty essential that this works again :S
I built it myself out of parts from eBay etc. about a year ago but only got it working about two months ago.
Any help is greatly appreciated

Try reseating the video card/memory.

If that doesn't work:
I would try unplugging your machine from power, holding down the power button for a minute, then plugging it back in.

Just done all three to no avail :'(
Still nothing but the fans whirring.

I've just been reading some other peoples similar (but not identical) issues on other forums and the end answer for them was a shorted motherboard. I'm praying this isn't a possibility here?!? :S

just spent an hour removing my only other functioning PSU from a dead PC and tested it in this. Still just fans :'(
I'm going to try my spare celeron d in it tomorrow, but i doubt it'll work :/

It sounds like a possibility. You said you've tried a PSU from a different system and you've tried reseating the video card and RAM but hadn't solved the problem, so obviously these are not the issue here. When you go to swap out the processors, check the back of the motherboard for any places it could short. To do that, first disconnect the video card and the cables (power, fans, SATA, USB, power and reset), then disconnect the motherboard from the case and check that there are no stand-offs in the wrong place (there should be only be enough stand-offs for the holes in the motherboard and they should line up perfectly) or a piece of the case where it shouldn't be.

ok, will do.
I'll report back in a few hours once I've done it.

right, motherboard is currently out of the case. I looked on the underneath of it and couldnt see anything that ooked wrong (but i'm not sure quite what to see). I swapped the processor, connected all critical cables and plugged the power cable in. There was the typical 'just recieved power' noise, but when I pressed the power button, there was a quick, deep sounding 'vrrt' and nothing else happened. I swapped in the old processor, same again. :S

after some fiddling about with it, i seem to have managed to get it back to the nothing but fans state (this happens with either processor in). any suggestions?

Well it sounds like it's time for a new motherboard, PSU, CPU, whole system.

Really from the sounds of it your system is dead and nothing you do to it is going to bring it back.