Login loop after OS X 10.8 update (WindowServer problem?)
I started OS X in recovery mode (http://support.apple.com/kb/HT4718) and chose "Reinstall OS X". This downloads OS X from the internet and reinstalls it. The download took me some 2 hours, installation another 40 minutes. Problem fixed. 

I was afraid this reinstall would completely format my drive, and install OS X from scratch. But everything seems still intact. My (and all other users) home folder is still there, 3rd party apps in /Applications are still there. And everything is still working.

I never heard of this built-in Recovery System in OS X. Good to know it exists.