Hi,
indeed, it's only for linux but you have another option :
http://www.amsn-project.net/forums/viewtopic.php?t=944&highlight=ml20It's a sticky thread in the announcement forum...
About the codec thingy, no, aMSN doesn't use the windows API to play the video, we use our own code to decode it.. yes, there is a codec, but it's only part of aMSN (needed to encode/decode webcam during a call), and the viewer just works the same way as when you are receiving the cam from someone over the net... the codec is not installed on the system and doesn't follow the win32 API for automatic detection by normal players (It's a Tcl extension).
Hope that helps.