Hello,
do you know that the entry for the first DNS on your client must be the ip of the PDC ? If your PDC isn't your gateway, you can add a foward in the DNS section to your Gateway IP
Try to ping the fully qualified hostname for example "ping zentyal.zentyal-domine.lan", first localy from your PDC and then from the Client-mashine.
... and do you know how you join a client mashine to a Domain?
... to join the mashine to the Domain you have start xp or win7 with admin rights and than change the workgroup to your Domain by
right-click COMPTER / go to Comtunername and change. Therefor, its the best to take the Administator user, where you first changed the password to one you know.
Then reboot your windows client and can connect a user to the Domain.