you can install the appropriate kernel using the .deb at http://packages.ubuntu.com/search?su...ubuntu-modules
installing this also give you the choice in the grub menu.
personnaly, I installed linux-ubuntu-modules-2.6.24-18-generic_2.6.24-18.26_i386.deb
and I was able at this point to install the nvidia drivers.
Computer are like air conditioners: they stop working when you open windows