Ran into this on a failing MP repair in our DMZ. The error was a 1603, and according to the mp.msi logs it was unable to create the virtual directories for the MP to function. What was looking like a complete IIS rebuild turned out to be a known issue surrounding BITS; and a far simpler solution then I had originally imagined.
The long and short or it; uninstall and reinstall the BITS feature, then reinstall the MP site component and bam.