Instant LAN software.

Hello to you all. i have a small internet cafe am running and i would like to communicate with my clients easily and conviniently when they are using the pc. Which program should i use thats eay for the general public to use? something thats not complicated for the public. Most of the users always shout can I have more time, I need to print. So i need to eliminate that by using an instant chat program. Any suggestions?