This is because Win 2k3 by default requires signing of SMB packets.
Windows 2003 requires SMB signing. Client side SMB signing has been implemented in Samba-3.0. Set client use spnego = yes when communicating with a Windows 2003 server.
So the best way is to update to Samba 3 and Set client use spnego = yes
You can also disable the requirement on the Windows Server if you have the authority, and accept the security implications, by changing this registry key
Windows Registry Editor Version 5.00
Just save that code as a .reg file and run it.
(Although you may find that Windows sets it back to enabled from time to time, depending on your other security settings)
But the preferred way would be to upgrade Samba