I was having problems a few months ago, and I ended up rebuilding my whole system after buying an SSD. I didn't have any problems for a few weeks, then I had the odd random restart, then it got more frequent until now it's back to its old tricks, restarting about every other day, sometimes as often as twice a day. Today was the first time I got a BSOD from it though, so I have something to report on - normally I get no dump logs, BSOD etc, just restarts back into Windows.
(Note that I'm only assuming it's the same problem as the restarting as the computer freezes for about the same amount of time before crashing - it could well be 2 completely unrelated problems.)
Windows 7 is...
Window 7 Enterprise 64bit
System came with Home Premium but I installed Enterprise as I have a VL key.
Age of the system is about 15 months old (with variably newer bits inside). Age of the OS installation is about 2 months, maybe 3 months.