Well, the simplest way would be to use the messenger service, but the problem with that is that if you have it enabled, you'll likely end up getting spammed via it.
From a pragmatic perspective, you should probably just leave a written note by the computer, or tell the person(s) in person.
how do i create a simple windows window with my own personal message in it so that when my roommate comes in to get on my computer i can tell him any useful information, and then he just clicks the OK button? do i just do this through command prompt?
i know that when there are multiple users on one computer, you can go to user accounts and message another user so that when they log on they are shown a window with the message. i want to do something like this but since this is my computer and i am the account on it, i want to be able to just make a window for the few times i know someone will be on it later.