Originally Posted by Deviousmind

Right click on My Computer and select properties.
Click Advanced tab.
Click Settings under Startup and Recovery.
Under System Failure uncheck Automatically Restart.
Click OK.

This won't cure your problem, but instead of restarting you'll likely get a Blue Screen of Death (BSOD) instead, the information displayed will allow you to trace your problem, or post it here!

If you still get the system rebooting and there is no BSOD then it's likely that your processor is overheating or your PSU is faulty. Try checking the system health in the BIOS for the processor or system temperature. You'd like it to be less than 50d Celcius, anything over 75d is running dangerous.