LAN Instant Messaging

We would like to implement an instant messaging solution for all the computers in the office. We have been looking for a program that would work for us communicating with each other, but would not allow people to talk to friends, etc in other places. However, we also want to be able to communicate with a couple of remote retail locations via the IM software.

Is there a good program that allows for LAN messaging while also allowing for connection from specific locations?

Setting up a VPN at the remote locations would be difficult because we have no IT people in those locations and they are spread out around the country.