You may have used the Net Send utility to send text messages to other network users from the command-line in Windows XP and previous versions. However, if you tried this in Windows Vista, you’d see it doesn’t work.
Net Send has been replaced by MSG in Windows Vista, available in the Business and Ultimate editions.
Here’s an example of sending a message:
msg /server:servername username "Text to send."
For help type:
msg /?