How's your memory?
I've got a machine that seems to randomly crash more often that I'm expecting. Usually when I'm in the middle of doing something (of course).
I've gone through the usual suspects - cables, HDD error check and defrag, Windows Update, Anti-virus etc and nothing obvious but still the crashes.
That left memory. Notoriously difficult to test.
Luckily as part of the Online Crash Analysis tools Windows offers a Memory Diagnostic tool that's simple to use and helpful in tracking down the problem.
If you're having a problem that doesn't seem to make sense check out the Memory Test utility and see if that can shed any light.
For my machine... one stick of RAM was bad and has now been retired with a shiny substitute coming out to replace it and (fingers crossed) everything seems fine...