Yes! You can send messages among Windows 7 machines and Windows XP machines on a LAN using freeware. It took a lot of searching, but I found an app that works from a command prompt and in batch files to send messages.
For sending messages, use Sent from WinSend.
For receiving messages, each machine needs to run
WinSent Innocenti.
Both are available here:
http://www.winsentmessenger.com/download/
I had hoped to use msg.exe on our LAN, but after trying all the recommended Registry tweaks, firewall changes, etc., I could only send messages to Windows 7 machines. The above freeware apps worked right away sending to Windows XP machines and to Windows 7
machines. They work on both Workgroup LANs and on domains.
WinSend is a more traditional Instant Messaging application that starts at about $12 and goes down in price based on quantity of licenses purchased.