I configured Samba 3.0.20bb (in a Debian Sarge server) using the same smb.conf file I was using in a Samba 2.2.3a-13 (on Debian Woody server) but the XP box doesn't let me join the domain. I also try different smb.conf files using some examples as base.
The XP box ask me for a user account with permission to join the domain. I use ferrocement/root (ferrocement is the name of the domain, and root is also defined as samba user) and, after a while I the the message :

The following error occurred attempting to join the domain "FERROCEMENT":
The parameter is incorrect.

Here is my smb.conf file :

;*******************section global*****************
[global]
wins support = yes
dns proxy = yes
time server = yes
netbios name = ferrolinux
server string = Ferrocement PDC running %v
socket options = TCP_NODELAY IPTOS_LOWDELAY SO_SNDBUF=8192 SO_RCVBUF=8192
os level = 64
prefered master = yes
local master = yes
domain master = yes
security = user
encrypt passwords = true
domain logons = yes
debug level = 2
log file = /var/log/samba/log.%m
log level = 2
max log size = 50
hosts allow = 127.0.0.1 192.168.1.0/255.255.255.0
logon home = \\%L\%U\.profile
logon drive = X:
logon path = \\%L\profiles\%U
workgroup = FERROCEMENT
add user script = /usr/sbin/useradd -d /dev/null -g machines -s /bin/false -M %u
add machine script = /usr/sbin/useradd -s /bin/false -d /dev/null %u
;*******************section homes*****************
[homes]
comment = Home Directories
browseable = False
writable = True
;*******************section profiles*****************
[profiles]
path = /home/samba/profiles
browseable = False
writable = True
create mask = 0600
directory mask = 0700
;*******************section printers*****************
[printers]
comment = All Printers
browseable = True
path = /tmp
printable = yes
public = no
writable = no
create mode = 0700