Broacasting a message to logged in users

What is the easiest/most effective way to broadcast a message to logged in users (W9X/NT)to let them know that the system is about to kick them out?

Our user policy is set up to log users out at midnight to enable backup, but despite education the help desk gets deluged with calls in the moring from users who’s files ‘no longer work’.

What we require is a message to greet them in the morning indicating that they need to log out and back onto the system.