This means the computer cannot find the boot file on the HDD or any other boot device like boot CD. Or the HDD has failed. Do as
JMPC suggested and make sure there are no USB drives, or Floppy or CD roms in the computer and boot the computer. If that fails, Next boot into Setup (Bios) and see if the HDD is being recognized. If so, then boot off the XP CD and get to the
Recovery Console. Here type
chkdsk /R and press Enter. The Check Disk utility will try and fix any file errors. If the HDD fails the check disk, then that drive needs to be replaced.