Compiz is the default window manager when you install Ubuntu. This guide will show you how to stop Compiz-Fusion from loading automatically on startup and make Metacity your default window manager.
Step 1: Run gconf-edit
Start Run Application by pressing Alt+F2

Enter gconf-editor into the box, hit enter
Step 2: Set Metacity as your default start up window manager
Once in gconf-editor navigate to desktop>gnome>applications>window_manager

under default replace:
/usr/bin/compiz with /usr/bin/metacity
it should like this when you're done:

*If you see /usr/bin/metacity after you navigate gconf-editor, don't worry. Compiz wasn't loading automatically and you don't have to do anything else.