Message to Users

I have just taken over nt admin in my company.The former admin was not a fan of popup notice to users incase of intended server service or reboot.I want to setup a popup to info my users if I have to schedule a server service or reboot or any quickmessage that I want users to read asap.Do I need to have messenger setup before doing this and what are the draw backs to my network.My older colleague is afraid it will affect network traffic and he is not in support of this.I will also appreciate a direction on how to set this up.