If you're looking for a good alternative, you could also try VLC, an open-source free player that can play pretty much anything that Windows Media Player can.
Personally, though, I would reccomend using Flip4Mac (which adds .wmv support to Quicktime). You'll also probably want to go to the DivX site and download the Mac codec, which will automatically add support for DivX movies to Quicktime (which accounts for a huge chunk of .avi files).