uninstall video drivers

How to proceed depend on what you want to do. If you want to try solving the problem, we need some more information, like exactly what isn't working, any errors you get and the contents of the file /var/log/Xorg.0.log. If you just want to go back to using your old drivers, you could just move your current xorg.conf to some backup location, as the version of Xorg in Ubuntu 9.10 is capable of automatically determining what drivers to use if its configuration file is missing.

I also second what Marc said about using the package management system. If you go to System -> Administration -> Hardware Drivers in your menu, that dialog should help you find and install the correct Nvidia drivers for your graphics card. That way, it is (1) easier to maintain your system, since if you use the installation script from nvidia.com you will have to rerun it every time there is a kernel update, and (2) it is easier to uninstall the drivers if you don't want them anymore.