After the installation, of course, we should set default-display-manager on ubuntu. There are two ways(not including all methods) to try:
1)Edit file "/etc/X11/default-display-manager", change display manager name in it and save, then reboot.
2)Run "dpkg-reconfigure gdm/kdm/xdm" to switch (gdm for gnome, kdm for kde, xdm for xfce), you will get a menu to choose. Of course you should do this as root, so add "sudo".
Refer to: http://ubuntuforums.org/showthread.php?t=205002