Hi and thank you very much.
Your advice worked well and the Nvidia's driver now flashes its logo and the Unity-desktop performs well too.
$ sudo apt-get install linux-headers-generic
$ sudo apt-get install --reinstall nvidia-current nvidia-current-dev nvidia-settings
$ sudo reboot
(The nvidia-current-dev package is my own idea...it's not really needed. ok?)
$ dpkg -l | egrep "headers-generic|nvidia"
ii linux-headers-generic 18.104.22.168.19 amd64 Generic Linux kernel headers
ii nvidia-current 304.51.really.304.43-0ubuntu1 amd64 NVIDIA binary Xorg driver, kernel module and VDPAU library
ii nvidia-current-dev 304.51.really.304.43-0ubuntu1 amd64 NVIDIA binary Xorg driver development files
ii nvidia-settings 304.51-0ubuntu2 amd64 Tool for configuring the NVIDIA graphics driver
Installation log attached.