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. This will not interfere with other Windows clients that do not support the more advanced security features of Windows 2003 because the client will simply negotiate a protocol tha both it and the server suppport. This is a well-known fall-back facility that is built into the SMB/CIFS protocols.