2. Weak failing Power Supply or lose Hard Drive power connector
3. Wrong IDE jumper settings. Solution; Reset the jumper/s on the IDE hard drive to MA or CS on the Primary master hard drive which is connected to the end of the IDE digital cable... (belt cable).
4. Failing hard drive: Solutions; Other than replacing the HD, sometimes if the Hard Drive is not detected you may get the "missing operation system" message... it may help HD detection if you modify the jumper configuration to an alternate setting... e.g. from MA to CS or CS to MA or even with jumper/s removed or according to jumper options in the Hard Drive label instructions. Modifying the BIOS > Hard Drives S.M.A.R.T to disabled may help a failing Hard Drive to be detected.
5. Lost partitions. Solution: A partition recovery application may help. Some of such applications can be burned to a bootable CD which you can run in unbootable computers, and detect/recover lost partitions. bootable partition recovery freeware
Other possible solutions:
a) Setting the Hard Drive as slave in another computer can sometimes help access the Hard Drive to recover files and personal data.
b) Even setting the failing Hard Drive as the Slave to a working IDE device (Optical drive or Hard Drive), can make the Hard Drive boot and/or help detect the OS. This may require disabling the HD S.M.A.R.T. setting in the BIOS.
No, the best way to fix this problem is to install the damaged drive as a second drive on a functional computer.
This may allow you to retrieve and back up the personal files on the drive. No, you do not back up the programs...only the files that you created...your photos, music, videos, documents, etc...
then you reformat the drive, install a fresh OS, and replace your old files back to the drive from your backup.
Your functional computer, used to retrieve your files, should have a professional antivirus installed. This is used to scan the damaged drive.
Don't depend on "repair" XP. You may need to re-install an OS, replace your files from backup, then resume using the computer.
OR have a computer shop do it for you. About $200.