You do not need the third paragraph that is about getting a root terminal on
your system without inputting any password (e.g., if you had forgotten it).
Since you managed to execute 'sudo passwd' and the problem is not solve, I
now see that it has nothing to do with the password.
It looks like your X server gets killed when you try to launch the desktop.
There may be relevant information at the end of /var/log/Xorg.0.log that you
can read from the terminal:
$ less /var/log/Xorg.0.log
You can as well copy (with 'cp') /var/log/Xorg.0.log to a USB key and show it
to us.
You can try to add a user (named "banana" in my example) and see if you can
graphically log with this user:
$ sudo adduser banana
