Sounds like the hard drive has got a problem.
This is an IDE drive, correct? If you have 2 IDE connectors (Primary and Secondary) on the motherboard, try switching from one to the other and then try to boot. Then switch back.
Double check your connections, try a different molex power connector, make sure the drive jumper is correct and not loose.
Also try going into the BIOS settings and "Restore Defaults" , or similar, on the Exit Tab.