I have a client who has recently started IM'ing, and is hooked. He wants to findd an IM app that he can use in his office for interoffice communications, but he doesn't like the ads that AIM includes.

I'd suggest Jabber to a larger company with an IT dep't to set it up, but this isn't that company. He's willing to pay a reasonable price for it, and it needs to run on Windows.

AIM compatibility would be a plus, if not a requirement. It doesn't have to be purely internal (meaning it can run on the AIM network if necessary), and ease of setup and use is key.

Suggestions?