Machine reboot

Hi,
I am facing a strange situation here. After installing Win2k Professional on my system, sometimes while booting, the system during booting prodcess, re-boots itself. This happens in loop and the only way to come out of it is to leave the machine for sometime (say an hour) and boot again. Then its fine.
As this is not happening regularly but sometimes, I can’t blame the overheating of system as culprit.
Any suggestions?