Thanks for the advice.
The CMOS is definitely reset and last night I tried booting:
* without a graphics card (same problem occurred)
* without RAM (same problem occurred)
So I'm fairly confident its the motherboard or the CPU thats broken, as I would expect the machine to stay on while...
I have a Gigabyte GA-965P-DS4 motherboard and the E6600 (2.4GHz core 2 duo). Last night I was fiddling in BIOS settings trying to find a setting that would make Linux recognise the IDE controller. During the process I inadvertently caused the machine to boot up with a clock speed of ~1.6GHz...