You are not alone. I have been seeing the same symptom. The system will
simply lock up without notice. This seems to happen every 3-5 days. The
machine is on 7x24, used in a business setting, and cannot be shut off. It
doesn't hibernate or anything.

My suspicion in my case is that it could be video related (Nvidia 7600GS). I
had hoped that the latest drivers would fix it, but not the case. I've loaded
the Microsoft DebugDiag tool to see if it can log anything the next time it
fails.

This has been a persistent annoyance since I installed (clean) the OS. If I
isolate the cause/solution, you can bet I'll post it.

Phil