In older versions of Ubuntu, the key sequence of Ctrl+Alt+Backspace for stopping the X server was enabled by default. Ubuntu 10.04 and 10.10 have this disabled by default. Here’s how to restore the keyboard command.
1. Go to the Main Menu, select System, mouseover Preferences and select Keyboard.
2. Select the Layouts tab.
3. Click the Options button.
4. Click Key sequence to kill the X server.
5. Check the Control+Alt+Backspace checkbox.
6. Click the Close button to close the options window.
7. Click the Close button to close the keyboard preferences window.