This blog post explains how to disable Compiz Fusion (3D desktop effects), and enable Metacity (2D) as the window manager for a GNOME session, for a single user. The solution presented here was tested on Ubuntu Karmic Koala, but it might work on other Ubuntu versions as well (Hardy, Interpid, Jaunty, Karmic and Lucid).
To disable Compiz and enable Metacity, run these commands in a terminal window (without the
$ gconftool-2 --type=string --set \ /desktop/gnome/applications/window_manager/current /usr/bin/metacity $ gconftool-2 --type=string --set \ /desktop/gnome/applications/window_manager/default /usr/bin/metacity
Logout and login again. Metacity should be running.
FYI the old setting was