Well here is a way to send instant messages to your friends over such networks as long as you have their ip or the name of the computer on the network. An easy way to find a computer's ip address is to type cmd in the run box on your start menu. This will start the command prompt. Type "ipconfig /all" with a space between "ipconfig and /all". That will list the computer's ip address along with other network specs. Now that you have the ip of that computer, you may send instant messages to it.
Go to the run command in your start menu. Type cmd in the box which will open the command prompt just like above. Type: net send "ip address" message. If the computer you were trying to send a "Hello" message to had the ip address of 127.10.1.1, you would type "net send 127.10.1.1 Hello"
Additionally, it is possible to use a wild card such as * in the ip address parameter.
Example: net send * message
The above command will send your message to every computer on your subnet. This is great for system administrators to send out messages to all windows computers on the network notifying them that you are going to be taking down the server or something. However, if you are a student and feel like pulling a prank at school by sending some message across the school network, you will screw yourself. With todayís zero tolerance policies in schools, you will be suspended possibly even expelled. Trust me; itís happening all over the USA.