Try to completely remove your nvidia drivers from your system:
sudo apt-get purge nvidia*
Remove your xorg.conf
sudo rm /etc/X11/xorg.conf
Reinstall xorg completely
sudo apt-get install --reinstall xserver-xorg-core libgl1-mesa-glx:i386 libgl1-mesa-dri:i386 libgl1-mesa-glx:amd64 libgl1-mesa-dri:amd64
Re-configure Xorg
sudo dpkg-reconfigure xserver-xorg
Reboot
sudo reboot
You should be greeted with lightdm, this will default everything x the same way a fresh install would.
After this you can try installing the drivers again using the 'Additional Drivers' tool in Ubuntu but if those drivers don't work you can test the latest drivers from the x-swat ppa
sudo apt-add-repository ppa:ubuntu-x-swat/x-updates
sudo apt-get update
sudo apt-get install nvidia-current
Note - if the above doesn't work - try also using the PAE kernel
sudo apt-get install linux-headers-generic-pae
(tip received by @Scott)
sudo mv /etc/X11/xorg.conf /etc/X11/xorg.conf.backup