This problem was brought up in an earlier post and the person solved it by replacing the ribbon connecting hd to motherboard. Maybe the Master Boot Record is corrupted. You could try rewriting the MBR from the install cd.
http://www.webtree.ca/windowsxp/repa...ot%20Sec tor:
You could also use a tool from your hard drive manufacturer to diagnose the drive to make sure. Flashing the motherboard would be the final thing I would try if the problem persists.