Windows 7 Unexpected full shutdown problem

I've had Win 7 Pro on my current machine (self-build) for 5 or 6 months now with no prior problems until this past fortnight when I've now run into repeated full shutdowns unexpectedly with no warning/error msgs prior to or on rebooting. Given how most other shutdown issues related to Win 7 do seem to provoke an error msg or at least a restart, I'm wondering if perhaps this is more of a hardware issue but given nothing's showing itself very obvious (i.e. no excessive dust, all fans working, temps ok, nothing loose or overheating, etc) I'm at a loss as to how to better pin down the likely culprit.

It's happened in all different settings - with 1 program running or 7, Firefox or IE, etc, so hard to pin it on a program, and to my (limited) knowledge, I've a vague notion that such problems do tend to be more hardware than software related, but would appreciate some additional input from all you smart people on here, who can hopefully point me in the right direction. Any utilities I could run that might help - I've already tried the temp checkers etc??