It could be a number of things...but it basically (IMO) comes down to the fact that the BIOS is not properly recogning the hard drive which should be bootable.
More detailed reasons include outdated BIOS, screwed-up MBR, or anything else which would disturb/prevent proper recognition of the boot partition/drive.http://smartcomputing.com/techsupport/deta...p;ErrorID=21747
I would not list the hard drive controller as a suspect...versus the drive itself...since the controller is on the drive, http://smartcomputing.com/techsupport/deta...p;ErrorID=21747
IME, XP will reflect error messages when the HD controller is noticeably bad. These can be found in Event Viewer.