With Ubuntu 11.04 the Unity Desktop came on my system. Not that I do not like it but I do not feel comfortable with it. Their fore I looked for a method to deactivate it right in the beginning.
If you just want to use the classical Desktop from time to time, you can select the desktop danager to logon with in the bottom task bar at the login.
To disable it permanently open System Settings. Do a left click on the logoff button top right corner look for Login Screen, open it and enter your password. Further select Ubuntu Classic as default desktop manager