LAN Messenger is a simple application intended to used for online messaging over LAN in windows. May be you have already use MS-DOS based Net Send messenger, but this one work in GUI and have many feature like using our own user name, IP based messaging and can do file transfer.
LAN Messenger is a decentralized UDP IM-style chat client which supports usernames, guaranteed-delivery IMs, broadcast chat, encryption, file transfer, simple UI, persistent settings, and IP blocking (sourceforge.net).Another nice feature is we can make a conference chat with all online LAN Messenger user. If some one in your LAN do a bad thing to you, you can block his/her IP address so they can't send you a message.
Simple, because LAN Messenger only 200 kb size and doesn't require install. LAN Messenger require .net framework 2.0 to get running otherwise error will occur with "The application failed to initialize properly (0xc0000135). Click on OK to terminate the application" message. I hope this information useful and happy messaging.