Happens to me every so often. 3 to 4 times a year. It's a PITA.
I usually have to uninstall, run regisrty cleaner, reboot, manually check registry for leftovers, reboot, reinstall. Most of the time it works. When it don't, I restore my daily backup.
Sorry I can't be of more help.
It just happened to Philly a few days ago.