On a Debian box you should be able to log into a recovery counsel from grub (at least you can on sarge). Just do that first and then run the Nvidia setup script.
If you don't have a recovery option, then add something like this to your /boot/grub/menu/lst file:
Code:
title Debian GNU/Linux, kernel 2.6.10-1-686 (recovery mode)
root (hd0,0)
kernel /boot/vmlinuz-2.6.10-1-686 root=/dev/hda1 ro single
initrd /boot/initrd.img-2.6.10-1-686
savedefault
boot
Or, just do a search--this question has been answered thousands of times.
