PC was playing up. With a boot CD managed to find out it was claiming the BIOS wasn't ACPI compatible. Nonsense, and some web forums suggested various reasons this might happen.
Ram problems was one, so I tried reseating it, and lost the video output.
Well, I did the shopping and dropped off some stuff at the church, then tried reseating the ram again and the graphics card as they're close together so I might have knocked it loose earlier.
Power on, and...
It caught fire.
Where can I get a PC with XP instead of Vista these days?