The four boxes I think giovanni is referring to are called "workspace Switchers".
If you are using Gnome desktop you can right click the panel (grey strip at the top or bottom of your screen) and select from the little menu "Add to panel".
At that point, a Window should appear from which you locate the "workspace switcher" icon and select it.
The "workspace switcher" should not be back again. You may need to move it into your desired position on the panel though. To do that right click the "workspace switcher", make sure it is not "locked to panel" and then right click again and select "move".
The above instructions are good for Ubuntu with Gnome desktop, however I assume they will be appropriate for most Gnome desktops regardless of the distro your using.