Throw in your XP/vista/whatever OS CD and boot from it. When it asks you what you want to do, load the Recovery Console and try the follow commands:
FIXBOOT - writes a new boot sector to the HD
FIXMBR - fixes the master boot record or replaces it if too damaged
then if no good try
chkdsk /f - will fix any issues with the file system
If those don't work you'll probably need to reformat the drive. It could be going bad but when I used to LAN my hard drives would get screwed up moving the PC to and from. I got these same errors but I could repair my boot sector with no problems.