Know nothing about nvidia cards so can't help there.
I would recommend booting to recovery mode as this will do some extra things during the boot process and then give you the chance to give your root password and work from there.
I would run;
dpkg --configure -a
May find some things not completely installed and attempt to install them.
If it does nothing you could try;
apt-get install --reinstall gnome
I assume you are using gnome as that is the default for Squeeze. That command should at least give you messages about the state and condition of gnome which is the meta package for the Gnome DE.
After running those hit Ctrl+d which should take you to a user prompt ($) instead of the root prompt (#).
and see what happens.
If you get errors there try rebooting to the normal menu entry. Then let us know what happens.