Can you set the PC up for a fall (i.e. play a wmv file!) and wait for it to crash.
Reboot the PC and go into Event Viewer (start menu - type Event Viewer into the search box, select Evetn Viewer from 'programs').
When the Event Viewer screen is displayed, look in the 'Summary of Admisitrative Events' box. Hopefully you will see that there are one or more of Critical, Error or Warning's coinciding with the time of the crash, i.e. last hour.
Dive down into the information displayed (click to expand, double click to look at specific info)You can't affect the operation of the PC by anything in Event Viewer so don't be scared to explore! Post back anything with a relevant date & time.
Feedback any info you can find. Below are a couple of examples from my system:
Faulting application Crysis.exe, version 126.96.36.19967, time stamp 0x471f94e3, faulting module atidxx32.dll, version 188.8.131.52, time stamp 0x49f7b54e, exception code 0xc0000005, fault offset 0x0000b937, process id 0x1468, application start time 0x01c9ed2a90ef3825.
System shutdown due to graphics card overheating