Try this;
* When at the login screen, press CTRL+ALT+F2.
* Log in by typing your username and password;
* Type cd ~
* Type mv .gconf .gconf.old
* Type mv .gconfd .gconfd.old
* Type mv .gnome .gnome.old
* Type mv .gnome2 .gnome2.old
* Type mv .gnome2_private .gnome2_private.old
* Type mv .metacity .metacity.old
* Type mv .nautilus .nautilus.old
* Type mv .gtkrc-1.2-gnome2 .gtkrc-1.2-gnome.old
* Switch back to the login screen by pressing CTRL+ALT+F7.
* Press CTRL+ALT+BACKSPACE. Don't worry, the login screen will come back in a few seconds.
* Log in.
With thanks to elreteipos at
http://ubuntuforums.org/showthread.php?t=395456