I am writing this in the event someone else encounters this problem installing Microsoft Office 2007 Service Pack 1 (SP1).
I’m running Windows Vista Ultimate x64 and was getting error code 78F whenever I attempted to install this service pack via either Windows Update or running the installer independently.
The solution is even more bizarre than the error: Place any data (i.e., non-music) CD in your computer’s disc drive and try again. You don’t need to actually run anything off the CD, just make sure it contains some files…a Vista or Office 2007 installation CD should work fine. Yes, it really works.
See the screen shot below showing the three failures and final success. Weird.