I log in as normal user and when i launch a software that requires gtk i get this error:
gabriele@gabrix:~$ sudo synaptic
(synaptic:6885): Gtk-WARNING **: cannot open display:
Does anyone guess why?

I never see that, however, could it be that user root has no permissions to use your X server? If so, in a terminal, as the user: xhost +localhost. This permits any 127.0.0.1 user your X server.

this is the result huuum !!!
gabriele@gabrix:/root$ xhost +localhost
xhost: unable to open display ""

Yes, that's to be expected. Permissions to use the X server must be given by it's owner. This should be the user that started it, not root.

Not positive it will work for sudo but when using su to go to root in a console window I have this in my /root/.bashrc.

So it could be worth a try you of course would change to your /home/user_name.

Still as normal user:
gabriele@gabrix:~$ ethereal
Xlib: connection to ":0.0" refused by server
Xlib: Invalid MIT-MAGIC-COOKIE-1 key

(ethereal:5048): Gtk-WARNING **: cannot open display:

as su root:
gabriele@gabrix:~$ su -
Password:
gabrix:~# ethereal

(ethereal:5095): Gtk-WARNING **: cannot open display:

i followed what you advised to do still get an error

Hmmm, I'm now at a loss to proceed, some of these other folks won't be, though.

try this:

$ echo $DISPLAY
:0.0
$xhost +
$su -
Password:
# export DISPLAY=:0.0
# ethereal

gabriele@gabrix:/root$ echo $DISPLAY

gabriele@gabrix:/root$ xhost +
xhost: unable to open display ""
gabriele@gabrix:/root$ su -
Password:
gabrix:~# export DISPLAY
gabrix:~# export DISPLAY=:0.0
gabrix:~# ethereal
Xlib: connection to ":0.0" refused by server
Xlib: Invalid MIT-MAGIC-COOKIE-1 key

(ethereal:9526): Gtk-WARNING **: cannot open display:

How come that you dont get the :0.0 or anything similar when you first echo $DISPLAY? hmm..were u in X environment or not?

ok, try this:

$ echo $DISPLAY

$ export $DISPLAY=:0.0
$ xhost +
su -
Password:
# export DISPLAY=0.0
# ethereal

I ended up formatting my machine and the news are that at the first boot it showed this message:
There was an error starting up the screensaver:

Failed to execute child process "xscreensaver" (No such file or directory)

Screensaver functionality will not work in this session.

Error activating XKB configuration.
Probably internal X server problem.

X server version data:
The XFree86 Project, Inc
40300001
You are using XFree 4.3.0.
There are known problems with complex XKB configurations.
Try using simpler configuration or taking more fresh version of XFree software.
If you report this situation as a bug, please include:
- The result of xprop -root | grep XKB
- The result of gconftool-2 -R /desktop/gnome/peripherals/keyboard/xkb ??? where this path comes from?
Anyway now i have
gabrix:~# echo $DISPLAY
:0.0
What does this all means??