I Believe that Ubuntu hosts the Nvidia drivers on their website. In startup you should see something that says "Push Esc to enter grub bootloader" or something like that. Push Esc and then select like console mode or whatever your option is. then let it load up. The other thing you can do is let everything load like normal then hit ctrl+alt+F2 or f3 or f4 or any of the f keys (they select which console you want to open.)
After you get into the console, login with your user name and password. then the command is
sudo apt-get update
sudo apt-get install linux-restricted-modules-2.6.20
That should be all you have to do, but if it doesn't work then you need to edit your x.org config file to accept the nVidia driver. To do this type
cd /etc/X11
sudo vi xorg.conf
After that scroll down to the line that says "Section "Device"" in there you should see a line that says Driver and in parenthesis it is probably "nv". You need to change that nv to nvidia. you do that by hitting "a" then erasing or adding what is needed like normal. After that push esc. Then hit Shift-Z twice that will save the changes for you. then restart your computer.
If that does not work you can get the source directly from the horse. here is how.
get in the console just like you did before. then type in
wget
http://us.download.nvidia.com/...x86-100.14.11-pkg1.run
after you download that, type in
sudo sh NVIDIA-Linux-x86-100.14.11-pkg1.run
follow the instructions through to the end. It should configure everything for you. you may need to get wget. to do that it is just
sudo apt-get install wget
Good luck again.
[edited, fixed linux for copying goodness]
[edited your going to have to right click on the link and copy it and paste it somewhere for future use.