Don't bother. Computer pranks are insanely stupid, try this instead. Here
Auto-replace various words in MS Word?
When i was at school our IT techs had a program that they would use to remotely monitor students (activity monitor or something), well we managed to get a licensed copy of it.
cue being able to remotely shut down, or send messages, or bring up websites remotley from computers on the network.
this was hilarious especially when a teacher would be using a computer on the projector, and you could bring up images of your choosing, or just shut their computer down. good times.