^^^ Check out
http://support.microsoft.com/default...;EN-US;Q330904 where it is suggested to use firewall to block net send messages from internet...
However, that said, I doubt if the firewall would be the problem as by default, I would guess that the firewall would allow LAN net send commands (since I use 3rd party firewall instead of WinXP Firewall, I am not sure)...
My guess is that the Alerter and/or the Messenger services might have been disabled... Possibly while using some software to maintain/improve the computer performance/security...
BTW, where is the response from the poster? Its almost a week and there has been no confirmation if any of the above worked...
Arun