If your PC restarts unexpectedly after briefly displaying a blue screen, then it's encountered a STOP error. If this keeps occurring you need to identify it.
In Vista and Windows 7 you can stop Windows automatically restarting from the Windows boot menu that should appear; if you use XP click Start, right-click My Computer and select Properties > Advanced tab. Click Settings under "Startup and Recovery" and untick "Automatically restart" before clicking OK twice.
Now when the STOP error occurs you'll see a blue screen with details of the error message; note down the description, any files it refers to, and the STOP error code. Then search the web for these terms to hopefully find a solution.