PC reboot itself

Hi, Im facing a problem. Im using windows xp pro, and pentium 4 1.8GHz processor. my problem is, my pc will restart by itself. so i suspected is the windows problem, than i re-install windows again. every things go fine after i re-install but if i shutdown pc and restart again and my pc will auto reboot every time when windows loading is complete. i try to change RAM, HDD, Power Supply also cant work but if i switch RAM, HDD, Power supply to other computer everything works fine. can any one tell me whats the problem?