Update to my problem noted above, plus a strange solution that worked.
Graphics card is fine, as are the hard disks. My windows installation had become corrupt, as I tested the Hard Drive in another PC and still had the PC reboot.
Okay then onto my attempts at booting from a CD. As I mentioned the PC would lock up and I'd get a few coloured pixels accross the screen. As recomended I removed and refitted my Graphics card, but same problem.
I can't guarentee this is a fix, but how I managed to get the PC to boot from a CD without locking up (strange colours) was to enable the USB Keyboard setting in the BIOS. By default this was set to disable.
When disabled PC would crash when attempting to boot from CD
When enabled I clicked 'any key' as advised and things worked as they should.
Plugged in an older PS2 keyboard to test and had no problems eitherway.
5-6 hours later I've re-installed all my apps and games without any further problems.
Not sure this will help jcs1987, as you may not even be using a USB keyboard, but hopefully you are and you get things sorted too.