Chat Messenger Clone (MSN)

I'm looking for a chat messenger software that can be customized for our site, that will allow users to download, install, and use the chat messenger system. A similar setup to MSN Messenger if possible, I would prefer to work with someone who has already done this before and can show a demo of their chat messenger. Basically, the user would register for an account, download the software and be able to chat with the other users with features similar to the current MSN messenger system. All the basic features like blocking users, appear offline, emoticons, status change, sounds, file transfer etc should be included with the messenger. We are also wondering if the user database could operate off of a free email script. For example all the free email account users, would be able to log into the messenger program using their email address and password. So basically the chat messenger would operate off the free email account's database. The email script we are thinking of using is [login to view URL] OR if you have a free email script we would be interested in using that if it would be easier for you to integrate with the chat messenger. Open to suggestions.