In 10.04 and earlier compiz was enabled by default but could be toggled on and off. You can either hit Alt-F2 and then enter metacity --replace or you can install the compiz fusion icon (this may have been called fusion-icon; it's been a long time since I was on 10.04.) This gives you a little icon in the system tray you can use to switch between compiz and metacity as your window manager.
In 10.04 and earlier compiz was enabled by default but could be toggled on and off. You can either hit Alt-F2 and then enter metacity --replace or you can install the compiz fusion icon (this may have been called fusion-icon; it's been a long time since I was on 10.04.) This gives you a little icon in the system tray you can use to switch between compiz and metacity as your window manager.