If you have an nvidia graphics card on your computer, the default Nvidia drivers installed when you install Ubuntu 12.04 on your desktop with the Nvidia graphics hardware, give a problem starting the GUI. Several AMD motherboards come with on-board NVidia graphics.
To overcome this problem, you need to install the 3 packages listed below:
Install them on your computer using the commands below:
sudo apt-get install nvidia-173 nvidia-settings screen-resolution-extra <enter>
Now restart your computer.