instant messenger crossplatform

Does anyone know of an instant messenger program that will work with both Mac and PC? The macs are running OSX and the pcs XPsp2. We want to keep this in house within our own network. iChat works for the Macs but not for the PCs. Is there something that will see and be able to messenger other computers inside our network and hopefully not have to run through any of our servers. the boss wants something that works basically like iChat does.
Thank you.