Comment 43 for bug 1283826

If you just want to try my fix skip to end of post.

I also stumbled up on this bug in the summer. It took me a while but I managed to solve it in the end for me. (Had the problem on 6 or 7 older computers.)
I was sure I documented it here as well but it appears I did not.
It was a while ago but I still think I can remember the correct issue.

For me the problem was in the user-session variable in lightdm.conf file located in "/etc/lightdm/".
I had set it to 2D to speed things up a long time ago:
"user-session=ubuntu-2d"
I had a slight feeling that I read something about it and it seams that 2D was indeed dropped in Ubuntu 12.10.
http://www.omgubuntu.co.uk/2012/08/unity-2d-removed-from-ubuntu-12-10

This page should probably be updated as well:
https://wiki.ubuntu.com/Unity2D

My fix:
1) Open file for editing. If you do not know how to do this then there is discussion here on how to edit system files (http://askubuntu.com/questions/92379/how-do-i-get-permissions-to-edit-system-configuration-files)
2) Change row "user-session=ubuntu-2d" to "user-session=ubuntu"
3) Reboot computer

Bob or anyone else, please report back if this helps you.