In the debian woody installation I created a user name called "quique".
When selecting user name into the Debian Logon Screen if I select the root user everythings goes fine, but If I use the "quique" user name, after entering the password, the logon screen appears again and cannot enter into the system.
I cheked this with "kde2", gnome-session and "failsafe" Session types

The "quique" user works fine If I use a text terminal

I don't know the answer, but maybe I can ask a question that the experts will ask later, to get the ball rolling:

How did you create this user?

This user was created during the Woody installation (the installation procedure asked if I wanted to add a user, I olny had to add username and password, nothing more)

I guess you mean KDM - the login manager, which is different than KDE (you can run them independently, anyway)

if you look in the kde control panel app, iirc there is a section for login manager, where you can set who can/cannot log in via KDM. you may also set access by searching for and editing a file called .kdmrc - location will vary per distro.

EDIT: you didn't specify which you're using, but there are at least two other login managers: GDM and XDM. I'm assuming they each have access lists, but I'm not familiar with them.