If you want to switch from the gnome-screensaver to xscreensaver on your Ubuntu 12.04 “Precise Pangolin” Box, it is recommended to uninstall the gnome-screensaver first and install the xscreensaver packages when the uninstall is complete. This is how you should do it:
1. Remove gnome-screensaver:
$ sudo apt-get remove gnome-screensaver
2. Install XScreenSaver and the additional packages:
$ sudo apt-get install xscreensaver xscreensaver-data-extra xscreensaver-gl-extra
If you change your mind and decide do switch back to the gnome-screensaver, you have to follow the two steps backwords:
$ sudo apt-get remove xscreensaver xscreensaver-data-extra xscreensaver-gl-extra
$ sudo apt-get install gnome-screensaver