MSG command in Vista (also NET SEND in XP)

Im having trouble sending messages across my network with this command. When I try the username to another computer on the network I get an error "username does not exist or is disconnected" and if I "msg *" I only recieve the message on my computer, and not the other in the network. Can someone post a someone descriptive guide on this command? Also, can this send messages to a computer with a differnet OS (such as Vista to XP or vice versa)?

messaging service does not have to be enabled on the target computer