XP keeps restarting when I try to shut it down

[Cincinnatus]

Today when I tried to shut down my computer it restarted instead. I haven't installed any new programs or hardware, and I've run virus scan to no avail.

I did a Google search to see if I could find a solution on my own, and this and several other websites suggested deselecting the restart on system failure option. This didn't work.

I don't get any error messages when Windows is shutting down, and I can hear the computer power off before it flicks back on.

What could be causing this? It's really, really annoying.