"- First I was using a graphical login: all user appeared on it (bart, root, httpd, mail, cdrom;etc !!!), "
This suggestion will probably fix part of the problem but maybe not the entire problem:
Log in as root.
Run kcontrol (Control Center)
Click on Systen Administration - > Login Manager -> Users
Then work on that page until you get the list of users displayed on the login screen correct.
Be prepared. Create a LifeBoat CD.