Boot fail

Hello,
i am abhishek i know few basic about BIOS one of my friend had som problem on his compaq pc he changed the cmos battery then his hard disk is not detecting . i went in to bios settings then in advance bios settings i made the hard disk to detect then installed the Xp then i restarted but again the hard disk is detected.When i change boot setting the Xp is starting well But when i off the system it condition is same it again showing boot failure.That is hard disk is not detected by comos what to do.