[ubuntu] default login screen changed to xterm, can't change back

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http://ubuntuforums.org – Does anyone know how the command in xterm, to open the login options screen? I accidentally chose xterm as my default session, and my computer logs me in automatically without asking for a password, so I'm stuck in xterm right now. The login screen I am referring to is the one in system > administration > Login Options (or Login something) in Ubuntu 10.04 lucid. Please help, I don't know enough about the terminal to do anything (I can open firefox, and some other things) Ubuntu 10.04 lucid lynx (Hardware)