That can happen if you are using Beryl or Compiz and it is not configured correctly. Happened to me once and I googled around and found a solution about editing a file to restore the default window manager, but now I can't find it again.
The other option is to uninstall beryl and/or compiz:
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sudo apt-get remove beryl compiz
then restart. That sould get you back to your desktop. You can always reinstall bery/compiz after you get your graphics card configured properly.