Well, I think David was right when he suggested that it sounded like a hardware/heat thing. I came home from work today and turned the computer on and it all just worked. So, I turned it off and opened it up and it was ridiculously dust free. Every other computer I've ever opened up has had dust but not this one (except for a tiny bit in the CPU heat sink and on the graphics card fan blades). The fans are all working normally too. None of the drives had any dust on them either.
So, I think it was heat (just for lack of a better explanation), but I just can't work out why it would overheat. Rebooting didn't seem to bother the bios, which has heat sensors and alarms. Drive overheating? Gamma ray burst? Anyway it's all working now, so all is good...for now. Thanks to everyone for your helpful suggestions. I really appreciate it. Cheers Dave
