I would be tempted to add a minimalist desktop such as Fluxbox and then totally
remove Gnome and re-install it from Fluxbox. Knowing that you are on dial-up
means that this is not something that you would do unless all else fails. It
could be done from the Live CD, if you change the repositories in Synaptic to
include the CDROM and uncheck other sources (Settings| Repositories| Ubuntu
Software tab in Synaptic). Later you can add them back and remove the CD check
in the box.

I would also consider deleting the .gnome and .gnome2 folders from your home
folder. You will lose your settings, but it is better than being frustrated
continually by menus that don't work. You could try this before you re-install
Gnome if you decide to go that route. Gnome should rebuild them when you log in.