Sometimes a difficult one to nail down, but usually a driver error... but can easily be an infestation, a failing hard drive, memory, video graphics card, cpu fan, power supply temporary intermittent failure.
You can start to narrow things down a bit by first turning off the automatic restarts, then looking at your event viewer to see what pops up at that time... then testing each device if it will stay running long enough.
To stop the automatic restarts, right click My Computer, Properties, Advanced, Startup and recovery, Settings, the uncheck the "automatic restart" under System Failure when it comes up.
Part of getting it fixed is to rule out the failures.
Run a scan for infestations with MalwareBytes, then with SuperAntispyware, then Avir Antivir antivirus. If anything bad shows up here, but you cannot read it, download and run HiJack This and run a scan, then ComboFix, and a scan... Then post both the HiJack This scan log and the ComboFix log here for the experts to suggest what problems they might detect.
Then check your hardware.
1st, check your Device Manager Start->Settings->Control Panel->System->Hardware->Device Manager. If no red or yellow flags, then go to step two
2nd, download and run the drive fitness test from the manufacturer of your hard drive... or you can use the one by Hitachi, Seagate, or Western Digital. Look first for the S.M.A.R.T. test showing up "healthy" or "unhealthy."
3rd.If it starts looking good, run the long extended test.
4th, download and run MemTest86. It takes a long time to ferret out some errors, so run for 4 hours or 7 passes, which ever takes longer. It it quits all together, then remove one memory module and run with the other. Then revers until all modules are tested.
5th, take another look at your device manager, and consider changing the driver, either to a more recent, or a more ancient one.
At that point, you should know a few things... get back to us with your logs, so we can talk thing over.
