Check to see if there is a timer set to restart. I think there is a scheduler in the sleep settings and maybe other places.

Also write down the time of the reboots and check the system processes history to see what was running at the time of the incident. Look for spikes.

open the event viewer and read though the logs. Should tell you exactly what was happening.

Secondary thought. For years I had a Win7 doing something similar. Eventually discovered one of the ram chips had a physical fault. Replaced it, poof!, problem disappears.

Thanks.
Memory issues - yup, and last time that happened it was on win xp.

I had similar issues in the past and it was a psu problem. The event viewer did help, but it's not particularly friendly.
The error message wasn't helpful being 0x0

Methinks, at a guess, it might be a failing psu again.

Memory issues. Those are the ones that most of the times bother you. :)

Interestingly, it hasn't crashed in almost a week.</tempting fate>

Just to update this thread with a possible problem, hopefully now resolved: I took the machine apart to check it over and I suspect it was overheating.
We had some building work done recently, and this included plastering work; notorious for dust.
I thoroughly cleaned out the internals and touched nothing else. No surprise there was a lot of dust trapped in all the important places.

So far, no crashes.

Would that all crazy things can be so easily addressed, after all, life in general is rather dusty.

I had an HP box with extremely loud fan noise. I opened up the back and found a couple handfuls of cat hair. And here I thought Muffy had just ran away.