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I just did a clean install of 12.04 and everything went fine. Then I started adding some software packages and was asked to restart so I did.
But then I got the error "failed to load session ubuntu".So after searching around a bit I found a post that said to do the following:
sudo apt-get update
sudo apt-get install ubuntu-desktop
sudo apt-get -f install
sudo dpkg-reconfigure ubuntu-desktop
sudo reboot
sudo apt-get install gnome-session
sudo apt-get install lightdm
sudo apt-get install unity-greeter
sudo dpkg-reconfigure lightdm
