If you installed Windows update KB885250 "MS05-011: Vulnerability
in server message block could allow remote code execution", try
uninstalling KB885250 in Start | Control Panel | Add/Remove
Programs. See
http://support.microsoft.com/?kbid=895900 "You
cannot save a file from your Windows XP-based or Windows
2000-based computer to a shared folder on a file server" for more
information.