Comment 7 for bug 1036830

(btw - when you fix the login problem, by taking ownership of .Xauthority, but if your 3D acceleration is still broken, you can log into Unity 2D successfully - but what happens when you try to log into Unity 3D, is you see your background, and desktop icons, but no launcher, no HUD, no menubar. You can get a terminal with ctrl+alt+T, and logout that way, to get into Unity 2D, and fix your driver install).