Computer shutdown and reboot automatically

My computer shutdown and reboot automatically without any reason. I try to remove the check mark (reboot automatically) on the computer advance setting but the problem remain. There are time, the computer keep rebooting without completing the windows booting. I tried to reinstall the Windows XP and I successfully completed the installation without shutdown problem. When I installed the office software the problem come back. I try to re-install the windows again and installation complete without any problem.