I think I must have caught something off Serenitys PC as I have just had the same thing happen to me as she described
The PC locked up completely,no hard drive activity and CTRL,ALT,DELETE wouldnt bring the task manager box up.
I had to use the reset switch.
I checked the error log and there wasnt anything in apart from a DCOM error at 13-50,time it happened was 10-58pm