i've done this in school...and it's freakin hilarious...try it at work for those of you who are older....
open command prompt (run cmd on xp)
type net send * your message
this sends your message to everyone on the network, using the Windows Messenger service (different from MSN messenger)
For individual messages, replace the * (known as a wild card) with the computer name.
You may want to change your computer name to hacker as it sends like this
(Your Computer Name) sends to (Their computer name):
Message
or something like that
*USE AT YOUR OWN RISK*