Could be power supply.
It could also be some other hardware issue or a hardware driver.
Does it flash a Blue Screen (BSOD) when it restarts? If so right click on My Computer and go to properties, under the advanced tab click the settings for startup and recovery. Uncheck the box for automatically restart on system failure. The BSOD will tell you what driver failed and you then have a starting point for internet research.
You can boot the computer and go into the BIOS screen and let it run there for a bit. If it reboots from there it is not a driver. Usually you can monitor temp and voltages from that screen as well.
You can also get an ubuntu or knoppix disc to boot from. If it still does it when booting from the disc it is probably a power supply or overheating issue.