[Samba] "smb_pam_auth failed -Rejecting user !"

[Les Moodley]

Hi,
 
Please could you assist, I am fairly new to samba, but I have managed to setup a samba server serving windows xp & 2000 clients, using the samba HOWTO doc. 
 
My problem is as follows,
I can connect from some xp clients using the command "net use o: \\server\username " 
But other other XP clients cannot, the samba log for the client "smb_pam_auth failed -Rejecting user !"
 
I am trying to connect with valid user names and passwd, I checked it with the "smbclient" command on the server. What could be different on these machines?
 
Best Regards,
Les