lightdm hangs on "Checking battery states" on Ubuntu 12.04

One day I got my lightdm hang on "Checking battery states" by no obvious changes to my Ubuntu 12.04 running on Virtualbox. In the first time I try to drop to the shell and type "startx", but a "DISPLAY can't be found" error was responded.

Then I searched for the solution on Google and tried so many solutions. Finally I got my lightdm working, but my qt creator 3.0.0 won't start and shows "Cannot initialize GLX" errror. Trying to start qt creator 2.7.1 and firefox is no problem.

One guy mentioned this in here, said this is because of the OpenGL. I followed his step to remove all nvidia related packages and try to reboot the machine. Magically the qt creator 3.0.0 can be started!

Here I try to record my steps for about hanging and GLX issue. However, since so many instructions I tried, it's not precise for my steps. Please notice this.

For solving lightdm hanging issue:

1. apt-get install gdm. (choose gdm as default display manager)
2. reboot
3. apt-get remove gdm
4. dpkg-reconfigure lightdm
5. reboot
6. (drop to the shell) service lightdm restart

For solving the GLX issue of qt creator 3.0.0:

1. apt-get remove --purge nvidia-*
2. reboot