has this just started happening or have you had hard disk replaced?
do you have more than one drive?
check you'r BIOS settings for the hard drive, ensure description and type correct, including boot sequence.
if more than one drive (which is always better for fault finding)
swop SATA/IDE cables over, if it works leave it
if not copy/save information to other disc, re-install OS
shaun c, November 2007