Run this command in the terminal.
Then do this again:
sudo mv /etc/apt/sources.list /etc/apt/sources.list.backup
If you do not receive error messages, then try enabling the nvidia driver. If you do, then, run this command:
Ok, go into System -> Administration -> Software Sources.
Check the first four boxes (if they are not already). Below it says 'Download From' choose 'Main Server'. In the second tab, called 'Other Software' uncheck everything. Then hit Close at the bottom. If any error messages show up report them here.
We might have to manually fix the sources.list file.
If it gets to that point I can help you further. I know how to do this in Debian, but not Ubuntu.
sudo mv /etc/apt/sources.list.backup /etc/apt/sources.list