Regardless of the scripting or programming language, the target PC would have to be "listening" or expecting the traffic. So for example, all of the PCs participating in this system would need to be connected either to a central source or have already established a direct connection with each other as in a peer to peer fashion.
If the target PC is not listening for this type of traffic, then it would have to check with the central system on a routine basis to see if any new data needs to be pulled down to itself. Then whatever application is running on the target PC, would generate the pop-up balloon in the system tray.
as I am not sure what you are trying to do here are a couple of suggestions.
Either use sockets
check out pusher.com. This is what I use for alot of things at the moment as it save writing a socket program and because you are using their API it only take about 10 minutes to get everything up and working.
As stated before the other computers will have to be logged in and listening for the event to be triggered.