This problem is quite common, mostly in the days of that dire OS they called windows 95, this was known bug or some called it a feature. If this is not OS related, then it's probably an issue with the BIOS that may need to be flashed. Can you tell us:-
What brand computer is it?
What Operating System and Service Pack?
How old the PC is?
What the BIOS and version is?
Regards,
David Chances
February 2007