Could be any number of things. I would start by checking the windows event log at around the time of restart and see if it wrote anything there. It should also let you know if it's restarting cleanly or just crashing. Or if it's shutting down from overheating maybe. That should hopefully point to a next step.

As others have said, check the system logs.

There's also a CPU temp monitor called core temp, available at http://alcpu.com/

Useful to see core temps, I think it does logging too.

Has it only happened once?

Historically, I had a callout to a place where the machine rebooted whenever a train went past: crappy PSU with a dry joint.

Also a machine with an underspecified PSU that couldn't provide enough power. A physical or optical drive being accessed were enough to tip it over the edge.

I know it sounds stupid, but are you sure it's not being caused by transient power outages? I thought this was happening to me until it happened one evening so I saw the light go off and on again at the same time and put two and two together.

UPDATE: Memtest ran all day without any problems; no issues found and it hasn’t rebooted during so I’m guessing it’s a software issue, rather than the PSU, right?

Could be windows update. Some updates restart your PC when you're not using it

In my experience random restart issues happen either during extreme load or when idling in ultra-low power. Check the event viewer logs after the crash. Run memtest86. Go from there.