Notice:This doesn't work on Windows Vista...
In this video I'll show you how to send a message that will pop up on an other computer using command prompt..
It is illegal to do this with a computer which you do not have the right, So I suggest you only do this with friends...
Here are some of the command
ipconfig=Check your IP Address
net view=Shows all computers connected to your network
tracert computername= Shows the IP Address of any computer connected to your network
net send * your message here=Sends a message to all computers on network
net send 000.000.0.000 your message here=Sends a message to a specific computer
help=Shows a bunch of commands