These days, now that OS's automatically manage IRQ levels, IRQL BSOD's are almost always due to a bad driver or bad RAM.
In your case, the NT kernal itself crashed, which indicates that something else caused the problem, and wasn't detected until much later on. Makes it harder to track down the exact cause of the crash.
To ensure nothing in the memory subsystem is at fault, run Prime 95's blend test. Assuming that passes, you almost certainly have a driver problem.