If I remember correctly, Ubuntu Lucid Lynx (10.04) and Maverick Meerkat (10.10) lacked the concept of a “control center.” Although the app is readily available, Ubuntu did not make use of it “visibly.” The control center remained hidden among the multitude of apps that comes pre-installed with the operating system. Natty Narwhal has the Control Center (a.k.a System Settings) prominently right below Shutdown in case you’ve missed it for some reason.
There is a reason for this, of course. The main menu structure supported by Ubuntu in the said versions made a control center redundant. Everything a user needs to do user-level and/or system-level changes are already listed under the main menu.
It is easy to miss things because of the confusing drop-down main menu in the previous Ubuntu versions that, frankly, I think is so Windows 95. Going back, it is easy to miss things let alone an app to set the default desktop user session that is named Login Screen.
To launch the app, if you are using the old menu system it can be found under System > Administration > Login Screen. In Unity, toggle Dash (tap the Super key or click the Ubuntu logo at the top left) then type in Login Screen, or if you want the long way it is under Dash > More Apps > Installed > See More Results > Login Screen (scroll down, the list is in alphabetical order).
Login Screen when opened becomes Login Screen Settings. Under the hood it is simply called gdmsetup. The said app requires administrator privilege for you to be able to make changes. What it does is it lets you set things like Play Login Sound, Show List of Users and a few more – things that are seen and applied at the login screen. Of course, we already know by now that this is where you change the default desktop session too as indicated in the image below.
The list will include all predefined desktop environments that your system has available. By default, the changes here will also be reflected in the guest session.
If you want the Guest Session to have a different desktop (make it Classic Gnome instead of Unity), edit the file: /usr/share/xsessions/guest-restricted.desktop
Change the line
Exec=/usr/share/gdm/guest-session/Xsession gnome-session –session=ubuntu
Exec=/usr/share/gdm/guest-session/Xsession gnome-session –session=gnome
Note that only the value after –session has been changed from ubuntu to gnome.