Send messages to other users, computers, or messaging names on the network using the net send command in Windows. To be able to use the net send command you must a Command Prompt window (Click Start -> Run..., type cmd and click OK).

Syntax
net send {name | * | /domain[:name] | /users} message

name : Specifies the user name, computer name, or messaging name to which you want to send the message. If the information that you supply contains spaces, use quotation marks around the text (for example, "Computer Name"). Long user names might cause problems when you use them as NetBIOS names. NetBIOS names are limited to 16 characters, and the sixteenth character is reserved.
* : Sends the message to all the names in your domain or workgroup.

/domain:name : Sends the message to all the names in the computer's domain. You can specify name to send the message to all the names in the specified domain or workgroup.

/users : Sends the message to all users connected to the server.
message : Required. Specifies the text of the message.

net help command : Displays help for the specified net command.

Another way to send messages to LAN computers is to use the program called Vypress Chat. Download it from here.