If you get the "BOOTMGR is missing" error in Windows Vista, you can fix this pretty easily if you still have your Vista install DVD. To fix this problem, insert the DVD, and reboot your computer so that it runs off the DVD. At the first prompt you get, click "Next". On the next screen, there will be a link in the lower left corner of the screen that says "Repair your computer". Click this link. It'll ask you to select an OS to repair. For most people, there is only one option here. Select it and press the "Next" button.

You will then get the System Recovery Option panel. You want to select the first option which is called "Startup Repair". It will then go off and repair the problem for a while and you will have to reboot (make sure you remove the DVD).

I did this and it got rid of the "BOOTMGR is missing" error that I had been getting.