LAN messenger for Windows and Mac with file transfer?

Can anyone recommend instant LAN messaging software between Mac and Windows platforms for using in an office?

We need to send messages and transfer files in our local network between Mac and PC machines.

Our company doesn't want to use the traditional online IM clients — Skype, AIM, MSN, etc., — for security reasons. My boss like something that would only connect members of the same LAN.

Thanks.