Check this site for information on some options for on screen keyboards for Linux. There is information here that explains how to use the on screen keyboard that is part of the Gnome GUI.
In 8.04 you can activate the Simple On Screen Keyboard by executing the following command at a terminal session:
If you want it to start automatically when you log on, you need to activate Assistive Technologies, this way:
Go to “System”->”Preferences”->”Assistive Technologies”.
Check the “Enable Assistive Technologies” option.
Click on “Preferred Applications”
Select the “Accessibility” tab
In the “Mobility” section select “onBoard” from the dropdown list, and check the “Run at start” option.
Click on “Close” twice.
Restart the machine.