LAN messenger for mac's and pc's

Hi guys at work we have been banned from using msn messenger from our head office in the UK, they are paranoid of virus on the PC’s. But if we can find a messenger that works on both pc and mac on the lan we are aloud to have one. I know there are PC ones out there but we need one which works with both mac and pc as our office has half and half. Does any one know of such a product?

Don't think yahoo will work as what we need is a system that runs over our network and not on the web.