Probably the built in SMART diagnostics telling you your HD subystem is dead or dying.
Before doing anything else back up your data if the drive is still accessible.
DO NOT perform any disk defrags or write any additional data to the disk. These actions will kill any chance of doing a data recovery from a bad disk.
Find the floppy or CD that came with the HD. it will have a diagnositcs program on it. Run that. If you lost the original disk then you can download a copy from the HD manufactureres web site.
It could be any of the following:
Bad hard disk (there is a controller built into it)
Bad MB IDE port (it also has a controller built into it)
Bad IDE cable / loose cable
Check that the IDE cable and power plug on the HD and MB are firmly seated in their connectors. If you have another IDE cable try replacing the one going to the HD with a different one. This will prrobably not help since the error message points to a controller problem.
The diagnostics should identify which it is and offer to print out an RMA fault report form so you can get a warantee replacement if it is the HD itself.
If it is the IDE controller you can move the MB end of the cable to the other IDE socket on the MB and try that. If both IDE sockets are dead then you need to find another computer and plug your HD into it and read off your data (documents, email folders, weblink folder, songs, videos, pictures, etc,)