Try booting it up in safe mode and leave it for a while.

If it doesnt reboot itself the it is more than likley a software probelm.

If it does reboot than chances are it is a hardware problem!

You could have a quich check to make sure your heatsink+fan are correctly fitted, check your memory is correctly fitted and check the CPU is fitted.

It may sound daft but I had a box in a few weeks ago that kept rebooting for no apparent reason, after trying a few things, I took the cpu out gave it a blow and put it back in, then I done the same with the memory and the computer was fine after that!

The most common hardware cause that I have come accross that cause reboots are incorrectly fitted fans or the wrong fan alltogether and harddrives being to close to something else and over heating!