Damnit!

My brother-in-law just brought his PC round. My plan was to tidy the innards and install a new HD. He'd been having trouble where by if he booted into windows - it would automatically restart. IF if selected F8 on bootup and chose "last known good configuration", it would work until he rebooted again.

We found out that his AV had ran out and used Avast to clean it up and it seemingly worked fine. Anyway, I've had the case open - swapped some cables etc and it just won't boot. It sits with the fans running but that's it.

I've reset the CMOS and re-jigged some cables to no avail. I'm not sure what to do next :/

Unplug the components one by one until you can get at least a partial boot.

When I first tried to boot it (tried a few times) I turned it off etc and realised a screw was loose in the far side of the case (behind the mobot tray where the case panel slides in). It shouldn't have caused a problem should it? It wasn't touching any components.

I shouldn't think so but I want to be sure...

Hmm is the motherboard properly on it's risers? It could be that whatever you've done with that screw is shorting somewhere it shouldn't.

Yeah the board is on risers. The screw has obviously been moved now.

If you have nothing on screen then I would remove the graphics card and reinsert it. Then try the ram and check the PSU motherboard power cable as that can do strange things (on some boards).

Can blow gently on the connection when it is removed.

If it was an IDE or floppy cable that you swapped, unplug it. Placing them the wrong way (which some cables let you do) can result in those exact symptoms.

Sadly I've unplugged them :/

so you have tried bare boot with nothing cept the CPU RAM & video card?

well if so all i can think is to remove the video card, see if you get the fault beeps from the bios, if its still dead then take the ram out and see if you get fault beeps.
all thats left then is to reseat the CPU. i dont think you get a bios beep without a cpu, but its worth a shot.

if all else fails take the battery off the board overnight and leave the reset jumper on the reset pins.

and make sure the little bios speakers plugged in?

and to add to this, the reset button isnt stuck in is it ? would cause the exact symptoms, fans going and nothing else as the reset is held down.

take all the cables off for hdd, power on, reset, led's and try starting it with a jumper or shorting the 2 pins with a screwdriver if you have a steady hand, or even start it with the power up connected and then yank it off so nothings on.

May also want to try the mobo out of the case as if it's shorting on the case then you can check the mobo tray and insualte the mount points with some tape.

does it beep?

did it ever beep?