but, actually im able to login as "root" in the graphic interface
Gnome i am assuming .
Also this sounds like you inherited this computer with "RHEL 5.something ( 1,2,3,4,5,6)" already on it.
If so the BEST thing to do would do a fresh install -- i do not even trust my daughter in using a second hand computer.
Who knows what is on it .
so during boot you get to the login gui window
it has your NORMAL user name then "other "
you are clicking on "other"
type in "root" as the user and then the root password
------- or ------------
is there a name called root in the login window
if so that might not be the real root account -- unless YOU SET THAT
login as the normal ( NOT ROOT) account
in gnome under applications/accessories
should be terminal
-- start the terminal ---
then type in
then as root
/* then the root password when asked */
that will just give a list of what is in the home folder