How To fix "BOOTMGR is missing" error

After installing Windows 7 and Vista with multiple hard drives installed, I often find this error after restarting the computer.

BOOTMGR is missing. Press <ctrl>-<alt>-<del> to restart

This is caused by another bootrec being present, usually on another hard drive than the one that Windows is installed on.

To Fix:

1. Disconnect all hard drives
2. Boot with Windows Installation disc
3. Select "Repair your computer"
4. Choose "Startup Repair"
5. Verify the computer boots without installation disc
6. Turn off computer and re-connect hard drive(s).