http://sahabm.blogspot.com/2009/04/change-ubuntu-log-in-screen.html
Search and download a GNOME GDM login screen. Cool Download Locations :-
http://www.customize.org/list/ggdm http://art.gnome.org/themes/gdm_greeter/ http://www.gnome-look.org/index.php?xcontentmode=150
Where to Install ?
System ->Administration-> Login Window ->Local ->Add -> xxxx.tar.gz -> Install -> Click on new theme -> Close.
