Aargh! The secrets of Ubuntu.
Ubuntu by default has disabled the display of all the startup applications that should appear in the Startup Applications system menu.
To display these programs, do the following in a terminal window:
cd /etc/xdg/autostart <enter>
sudo sed --in-place 's/NoDisplay=true/NoDisplay=false/g' *.desktop <enter>
Now, when you go to Startup Applications, you will be able to choose those you wish to enable or disable as illustrated below:
You can now stop those you do not want to start anymore.