Hi Mr Lee,
What make and model hard drive is dos installed onto was it the original drive from the HP ?
Have you checked the bios hard drive configuration is correct. if it is set in correctly you may be able to access files but not boot.
If the above is correct then is the hard drive working in the old days hard drives used rest the heads on the first cylinder of the drive where the boot files were stored. If there is difficulties reading this area then the drive won't boot.
You could use this from a floppy disk
http://hddguru.com/software/2005.10.02-MHDD/To test the hard drive.