Boot the LMDE LiveCD.
Open up the partition for LMDE - navigate to /etc/X11/xorg.conf
From there, open up a terminal (I think you can right-click in the folder and it's an option) and edit the file as root, gksudo gedit xorg.conf
If there is no option to open a terminal from that folder, you need to manually navigate to xorg.conf
Once that file is open for edit, change the line Driver "nvidia" to Driver "nouveau"
Reboot, if it works, make sure you follow the instructions
for Update Pack 3.
Then follow this guide
to set up nvidia drivers properly