The nouveau driver should be installed by default if the nvidia drivers are removed, but based on your description, I'm not sure what graphics drivers are currently in use. Could you issue the following command in a terminal window:
Also you could launch synaptic and search for "xserver-xorg-video-nouveau" which is the open-source driver. I would expect it to be already installed. I would suggest also searching for "nvidia" to see if anything with nvidia in the package name is installed.
lsmod | grep -e nouveau -e nvidia