If Vista Is Missing in your Vista Computer.

If you frequently get the following message while booting Vista:

Missing Operating System
or
Operating System Not Found

and you need to boot with the disk, and repair, here is what you can try.
Boot the Vista DVD, choose "Repair"
Open a Command Prompt.
Enter the following commands:

bootrec.exe /fixmbr
bootrec.exe /fixboot
bootrec.exe /rebuildbcd

Hit Enter and Reboot.

This should hopefully, have made your problem go away !