Hi, I just recently configured my samba server as PDC in ubuntu server 10.04 with this global settings:

[global]
log file = /var/log/samba/log.%m
passwd chat = *Enter\snew\s*\spassword:* %n\n *Retype\snew\s*\spassword:* %n\n *password\supdated\ssuccessfully* .
socket options = TCP_NODELAY
obey pam restrictions = yes
domain master = yes
map to guest = bad user
hosts allow = 192.168.0.
encrypt passwords = true
winbind trusted domains only = yes
passwd program = /usr/bin/passwd %u
passdb backend = tdbsam
dns proxy = no
netbios name = ubunutuserver10
server string = %h server (Samba, Ubuntu)
unix password sync = yes
workgroup = FORTALEZA
local master = yes
preferred master = yes
os level = 20
syslog = 0
security = user
usershare allow guests = yes
panic action = /usr/share/samba/panic-action %d
max log size = 1000
pam password change = yes
domain logons = yes

When I try to add a windows XP machine to the domain I go to My pc-->properties-->pc name--> and add the FORTALEZA domain with user root, the message "welcome to FORTALEZA domain" appears, but when I restart the PC and try to join the domain immediately pop ups the message "the FORTALEZA domain isn't available"

The user is added in samba with same passwords, the pc is added also in samba, I don't know what I'm doing wrong.
Note: I can enter all my samba shares without a problem with my local account, the issue is only when I try to enter to my domain.
I hope you guys can help me.

Maybe the typo in your hosts allow section? 192.168.0 =:]