I have an HP laptop. The Operating system that's running on it is XP pro. The problem that I'm having is when I try to sign in using my user name and account, the laptop would restart itself. I tried using the guest account and try to log in but the samething happens. Before this incident had occur, it was working fine. Now every time I tried to log in using my user name and account, and even the guest account the laptop would just restarts itself from the user screen name. Any ideas on how to fix this?

Try booting to your XP CD and using the Repair option.

This is a fairly common problem. It usually points to a corrupt Windows installation or a hardware error. Since you get to the login screen, it is most likely not a hardware error -- but that's not a guarantee. Follow ScottGem's recommendation. If that doesn't work (it does less than 50% of the time), you will probably require a clean reinstall of Windows. If that fails, it's probably a hardware error -- could be disk, memory, CPU.