I am having the same error msg. after two programs running together did not work as they should.
"Bootmgr is missing.

Press Ctrl+Alt+Del to restart."
The loop repeats indefinitely infinitely.
After thorough Web research it seems:
1. You need Windows PE 2.0 to get booting.
2. In order to do that, you'll need to d/l Windows AIK (992 MB).
3. According to MS, Windows PE will boot up a system, run a lot of Vista programs, and run a few utilities that came with PE.
4. Diskpart.exe is the utility you want to use in PE according to someone on TechNet.
5. Also needed, is a .IMG converter to process the 992 MB file.
I hope to get the Vista DVD/CD in 3 days but until then I will do the prev. steps once my d/l finishes it prob. 3 hrs.
This experience is a little frightening.