Right, even more weird revelations.

The problem appears to be sorted!

I actually did TWO things when I installed the USB HDD. I plugged that in and let it install, and I also re-routed my flash card reader. All I did was put a USB extension cord (that I have used for ages with loads of different things mind) between my flash card reader and the USB hub it is plugged into, so as to be able to route the card reader to the other side of my keyboard.

But, for some inexplicable reason, the PC does not like to boot when the card reader is plugged into the extension cable!!!!!!!!!

I unplugged the cable, plugged the card reader into the hub directly, and it booted absolutely fine. Just to check I left it off all last night and all today (as normal) and just booted up like 10mins ago - absolutely fine.

So the cause of the problem appears to be the extension cable.

So the net question : How in the fuck could an extension cable be a problem?!?!?!?!?!?!