Ubuntu Natty: Desktop freezes after login
Solution 1:
You can try to reinstall some packages without installing the whole system with the following:
apt-get --reinstall
You could also try removing the whole home directory of the user and creating a new one.
Solution 2:
As I said, it was the Nouveau driver, and the fix was to not use Unity in the latest-and-greatest version but its 2D stepsister, http://ubuntuforums.org/showthread.php?t=1728379.