It's ok now. I shut down the computer in the hope that I might get lucky after restarting, and I kind of did. I tinkered around the bios settings but didn't save any changes. While rebooting I got the blue screen of death (shock horror
) and then when I restarted it again it did a system recovery and now I have use of all four cores, hopefully permanently. I'm sorry I can't explain it more fully for anyone else who might get this problem, but I'm just not sure what happened.