For some people (like me) the new default Unity desktop in Ubuntu doesn’t look easy to opertate. However the Ubuntu is trying to differentiate itself with a distinctive to improve user experience (Ubuntu is rocking!!). Ubuntu 11.04, is using Unity as its default desktop interface instead of the classic GNOME.
If you’re running Ubuntu 11.4 and don’t like to use Unity, you can switch back to the old classic GNOME interface as shown below.
It’s actually pretty simple to choose between Unity and the classic GNOME user interface…
 During login time, Go to your logon screen, select the user you want to log in as.
– At the bottom task bar you can choose between different modes. Select “Ubuntu Classic”.
– Log in as normal.
* Another way to change theme is..
 Go to your (click on) “logout” button on top panel, select the “System Settings”..
– This will open “Control Center” then click on “Login Screen” setting tool.
– Where you can choose different themes. Select “Ubuntu Classic”.
NOTE- Please unlock before changing the setting ie provide your password