The following information was taken from the Samba FAQ, this might explain why:
Samba 2.2.0 + Win2K SP 2 or WinXP
With Service Pack 2 Microsoft have added some new
RPC calls and uses them to join the domain, the same
RPC calls are found on Windows XP Beta release.
Samba 2.2.0 does not have this new set of RPC calls
and this make problems to Win2000-sp2 and winXP.
Samba Team have yet fixed this bug in CVS tree and
it will be solved in samba-2.2.1 for both Win2000-sp2
If you upgrade your SAMBA package to a newer version, this issue should be resolved.