[Samba] Can't connect XP to Samba domain: (user name could not be found)

[Steve A]

I have done all I can think of to make Samba be a PDC, but I can't get my 
Windows XP-SP2 client to join the domain.  I've already edited the Group 
Policy for the roaming profile thingumyjig.  I am asked for the user/pass (I 
use root) to add the machine to the domain, and the error is:

"The user name could not be found".

Here's my smb.conf, and group information.

=================
[global]
	workgroup = SAMBA-DOMAIN
	netbios name = SAMBA
	server string = KJN Server
	smb ports = 139
	socket options = TCP_NODELAY SO_RCVBUF=8192 SO_SNDBUF=8192
	load printers = No
	show add printer wizard = No
	domain logons = Yes
	os level = 65
	preferred master = Yes
	domain master = Yes
	ldap ssl = no

[netlogon]
	path = /export/netlogon
	browseable = No

[profiles]
	path = /export/profiles
	read only = No
=================
Domain Admins (S-1-5-21-1171369666-3077905698-890339982-512) -> wheel
Domain Guests (S-1-5-21-1171369666-3077905698-890339982-514) -> nobody
Domain Users (S-1-5-21-82349369-3985415093-3734604251-513) -> users
=================

Obviously root is a member of wheel, so I would have thought this would 
allow me to join XP to the domain. root is also defined in 
/etc/samba/smbpasswd, with the same password as the system root user.

There seems to be a lot of info on Samba 2.x and PDC, but not Samba 3.  Does 
anyone have any ideas?

Many thanks,
Steve :)