I know there are many threads about installing Unity and repairing Unity installations and I have looked at several of them, but I haven't yet found an answer that clearly meets my situation so I'm hoping someone here can provide one.
I have an elderly Toshiba laptop on which I have (single boot) Ubuntu 12.04 LTS. When I first installed it I found that the computer's graphics capability is not up to running Unity 3D and Compiz, so I abandoned Unity and installed the Gnome desktop instead. That runs fine and I have no problems with it (I currently have Gnome 3.2).
Recently I thought I should give Unity 2D a try (in appearance it's very similar to Gnome of course). When I select 'Unity 2D' at login, I get the desktop and the top panel OK, but there is no Dash and I can only start programs from the terminal; moreover there is no top bar and no border to the windows that open with programs in them.
I worked out that that's due to Metacity not functioning as the window manager -- and sure enough, if I log in to Unity 2D and then start Metacity in the terminal, everything is fine: Dash appears and the program windows behave normally.
So the problem seems to be that when I login to Unity 2D, Metacity isn't starting as it should. Can someone tell me how to cure it? (Incidentally I want to retain Gnome as a working desktop environment: it would just be nice to have Unity 2D as an alternative.)
Thanks in advance