Do my users have to download the software to their computer in order to use the instant messenger or do I just upload the application onto my server and they access it once they are on my site (without having to download anything) ?

Also, I want an integraded file sending and video / audo conference ability in the latest version of the instant messenger. Is that something you can do?

They have to download the client that you create with the client creator and run it on their PC.

The WWIM uses NetMeeting for A/V.