It does it continuously, or once in a while ?
If it's the first, you probably got a virus/hardware problem.
If it's the second, you've got some software that's causing your operating system to crash (blue screen) and you probably haven't set the right options to show the BSOD instead of restarting, go check C:\Windows\minidump, if there are any dump files in there, post them here, they need to be debugged to find the root of the problem.