Fixing "BOOTMGR is missing" Error While Trying to Boot Windows Vista/Windows 7

If your Windows Vista computer has encountered the dreaded “BOOTMGR is missing” error, you aren’t alone. If the problem isn’t hardware related you can fix it with relative ease as long as you have a Windows Vista/7 DVD handy.Boot off the Windows Vista DVD and then click Next at the first prompt:You should see a link in the lower left-hand side for “Repair your computer”The installation process will attempt to find your Vista installation directory. If there’s more than one in the list, select the correct one and then use the Next button again.Click the first link for “Startup Repair”

The installation process will repair the system, and then prompt for you to reboot.
Alternatively you could have chosen the Command Prompt option instead, and use the following command to fix the boot options only:

bootrec /fixboot

At this point you should be able to reboot your computer, remove the DVD and everything should be working.