It seems like Google is seldom a friend when searching for botched SP1 installations, regardless of which stage it happens. You'll see a lot of incidents, but seldom any solutions...
Given the time and headache, I'm wondering if it's better just to back up your data, format, then IMMEDIATELY install Vista SP1 afterward.
Granted that doesn't explain WHY it happened, but at least it gets you back up and running.
I wouldn't try a repair-install at this point. Your files are on SP1, and repair-installing would cause even more problems trying to bring the OS back to RTM stage.