I have experienced some problems with samba,w2k and xp.
I may have got this wrong, but I'm pretty sure I read somewhere that the network authentication in w2k and xp differs from the ones in earlier windoze versions.
One solution that worked for me, was to disable encrypted passwords on the samba server, restart it, run the win2k-plain-password.reg file that comes with the samba documentation, and restart the w2k or xp machine.
Or it might just be enough to do a smbpasswd -a user password on the samba server..
