My desktop says error loading operating system when I turn it on. How do I fix this? I have tried to reinstall windows but it says there damage and it cannot be installed properly.

New hard drive time as it sounds like your current one is dead.

What is the exact message you get?

You can always try a repair installatation

Repair Installation
A Repair Install will replace the system files with the files on the XP CD used for the Repair Install. It will leave your applications and settings intact, but Windows updates will need to be reapplied.

Perform a Repair Installation

The exact message is error loading operating system when I turn on the computer this is what it goes to after it gives me the option of pressing F2 or F12

Is it a Dell?
See if you can boot the dell diagnostics partition. {F12}, then select diagnostics, once you have the diagnostics available run the memory tests and the hard drive test.

I would go with the repair installation