I installed 12.04 from CD in a partition where 8.?? had been working. The installation appeared fine but Ubuntu booted to a black screen. After updating packages and fighting with nvidia drivers for a while I now have a login screen.

If I log in using my password the screen goes black for a few seconds and then returns to the login screen. If I choose the guest session I get a black screen with a mouse pointer.

Ctrl+Alt+F1 works and I can log in, but many of the suggestions in previous answers either have no effect on the problem or fail with messages like:

Client failed to connect to the D-BUS daemon

Invalid MIT-MAGIC-COOKIE-1

X11 initialization failed

Is there a known way to get from here to a working system? I'd like to avoid reinstalling because my Internet connection is slow, and the original installation seemed OK.