PC Rebooting

The problem I have is this: Everytime I switch on my PC, I can only stay logged in for at least 5 – 10 minutes. After that, my PC reboots. After the reboot, it doesn’t reboot again. However I recently replaced my 20GB Hard Drive on which Windows was installed with another 20GB, thinking it could be a boot sector error. But my pc still reboots. What could be the problem?