10.10 to 11.04 classic desktop problems

I upgraded from 10.10 to 11.04. I'm suffering the same sort of problem identified in question 154794, (although they went from 10.04 to 11.04).

After download and install of the upgrade, when starting up I was told the hardware did not support Unity and it would therefore revert to the classic desktop, which in theory was fine. The system then started, but the screen was blank. When I clicked on the screen, patches of colour and images appear underneath. You can fill the screen with the correct desktop in this way, just by clicking all over the screen. When you try to open a menu item (Applications etc), the menu flickers and you can only see individual menu items for a few seconds. If you manage to load some software, clicking on the software (e.g. Firefox) window will show the picture of the desktop wallpaper. If you manage to close the software, a faint image of the software window is still visible on the desktop (almost transparent).

I tried using a Live CD version of 11.04 to check whether the original upgrade was faulty, and the same problem arose. The machine is an elderly Dell desktop (Celeron 2.33Ghz, 2Gb Memory).

Does anyone have any suggestions as to how to fix this, or am I simply going to have to do a completely clean re-install of 10.10? I'd obviously rather not do that, or it will mean reinstalling extra software (LibraOffice etc) that I had running under 10.10. If the problem is only within the desktop, is there a way I can load Kubuntu or Xubuntu over the top and keep the underlying extra software/data etc?

Thanks

David