Hi Antonio,
You should take a look at the /etc/inittab file. It control everything in bringing up a linux system. There is a line in this file concerning run level. It will be set to 5 in your case. If you change the run level to 3, linux will boot to a non-graphical login screen. You can login to linux and then type:
gdm
This brings up the GNOME DESKTOP MANAGER.
If you type
kdm
This brings up the K DESTOP MANAGER
If you want to boot automatically to the GNOME DESKTOP MANAGER, you will need to trace the bootup code in the /etc/inittab file to see what need to be changed. My system automactically boots to GNOME and my /etc/sysconfig/desktop file has:
DESKTOP=""
Also, from the /etc/inittab file, take a look at:
/etc/X11/prefdm