3) I already tried removing everything but the CPU and PSU. Is there a reason why you're suggesting these specific items? I could try the video card and 1 stick of RAM but I don't have a floppy drive.
-Deozaan
Some motherboards fail to boot without memory installed - you will just get beeps. A graphics card is useful if you want to see any error messages

. You could try known good graphics and memory cards - at least then you isolate that the problem is not the memory and graphics.
I suggested a floppy because you can boot up a floppy of diagnostic tools - or at least a DOS disc. If you don't have a floppy I suppose a single CDROM would do.
If it doesn't boot with a very minimal set of components it is one of three things:
- PSU - try swapping it for another one and see if things improve
- BIOS or NVRAM corruption. Most mobos require that you you not only pop the battery but also move a link to ensure the NVRAM is flushed. Check the mobo manual. Don't forget to put the link back before restoring the battery though or powering up as you will fry your board.
- Motherboard component is faulty or dying.
Good luck - it is a real PITA when this sort of thing happens and a good reason for buying a mobo next time that has a sensible documented BIOS (such as AMIBIOS) where at least you know what the beep codes mean!