Operating System not found

This may be a dumb question, but…it has been a long time since I’ve had to deal with this! My “old” desktop on boot after POST displays a message “operating system not found.” I can’t find my original CD (Win XP Pro). Can I install another copy over the old operating system or will I have to reformat the HD first? I really don’t want to do that…need some files there badly. Thanks for helping out an “old” computer analyst again.