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.