[Looking for a internal chat for comunicate varius MACs and Vista laptops]

Hi,

I am looking for a program that allows to chat for a little groups of pcs and macs in a enterpraise network, It's for communicate just a few laptops. Simple, not to sent archives nor videoconference, it's people that are really near but they want not be speaking loud in the office.

I try google and search in the forum, at frist sight I haven't see anything...

Thanks in advance