Okay, I kind of solved this. Rebooted, went to command line, ran the following command on the run file:
sh /home/mark/slackpacks/nvidia/NVIDIA-Linux-x86-96.43.23.pkg1.run --uninstall
I also could simply have run the command "nvidia-installer --uninstall"
I then installed the nouveau driver, but this did not work. The xorg.conf file was still the same -- it still referred to the nvidia driver. However, I saw that there was an xorg.conf-vesa file, so I changed the name of this to xorg.conf, rebooted, and it did work. The desktop fonts and resolution are a bit different, but otherwise it seems fine. Also 3d acceleration seems to work (according to glxinfo).
ETA: I went back to the proprietary driver, since it seems to work a little better. I have plenty of issues with my old computer, so I was experimenting with changing this driver to see if it would make a slight improvement, but it didn't. I think I just need a new computer.