Brodacast Messages from Linux Server to Windows Host

Oh ;)

You can use samba and smbclient to do this:

Brodacast Messages from Linux Server to Windows Host

cat message.txt | smbclient -M COMPUTERNAME

I don't think you can do a workgroup-wide broadcast message; you have to
specify each windows box in the workgroup by name, but that should be easy
enough with a simple script.

How do I send a pop-up message?
A. The syntax of the net send command is:

net send {name | * | /domain[:name] | /users} message
To send a pop-up to all users in your domain, type:

net send /domain This is a message.
Note Windows NT-based client run the Messenger service by default. Other
Windows clients must be running Winpopup.exe to receive the message.

net send sriram hello - will send hello to Sriram, if he is logged
on.

net send /users hello will send hello to all users that are connected
to your computer.