This is also a very common problem in many systems. Again, troubleshooting this problem is not too easy. The culprit could be your hardware also or software also. So lets see how to identify the culprit.
If your PC keeps restarting after loading Windows, then the problem could be Virus, Spyware or system file corruption. Sometimes this problem could even appear because of some driver problems. To trouble shoot this problem, first try the recovery console. Restore it to a previous date. If you think that the problem could be a virus, than scan your PC with the help of a bootable antivirus CD. I recommend using Avira AntiVir Rescue System. Download it from here. A quote from the official site
The Avira AntiVir Rescue System a linux-based application that allows accessing computers that cannot be booted anymore. Thus it is possible to repair a damaged system, to rescue data or to scan the system for virus infections
If you are facing BSOD (Blue Screen of Death), then the problem could be of a hardware malfunction also. Check your RAM first. Sometimes, you will also need to swap your PC parts from other working PC’s.