Make sure your /etc/apt/sources.list has contrib non-free If it doesn't edit the file as root.
su
rootpassword
nano /etc/apt/sources.list
It should look something like:
deb
http://http.us.debian.org/debian/ lenny main contrib non-free
deb-src
http://http.us.debian.org/debian/ lenny main contrib non-free
Make sure you save then in a terminal as root do: (you can copy+paste the entire command)
Code:
aptitude update && aptitude install module-assistant && m-a a-i nvidia-kernel
After successful module compilation, still as root do:
Code:
aptitude install nvidia-glx nvidia-xconfig && nvidia-xconfig -o /etc/X11/xorg.conf