If you can get a command line in recovery mode - you can edit xorg.conf to load the vesa driver (which is the default before installing drivers) instead of the nvidia one.
Change the value of the Driver from "nvidia" to "vesa" and try to reboot your computer and see if X loads
# make backup of xorg.conf
cp /etc/X11/xorg.conf /etc/X11/xorg.conf.bck
# edit xorg.conf
You can also do apt-get purge on the nvidia packages
I'm not in front of my ubuntu machine right now so the instructions are a little offhand - still, I hope it will do...