The latest incarnation of Flip4Mac is awful though, it seems to go through some sort of longwinded rendering process that takes nearly as long as it takes to play the movie. The older version used to delay opening the file for a moment, the new version opens instantly but renders while it plays, it means you can't skip forward through the movie if you want to.
VLC player is worth downloading, its a scruffier interface than Quicktime it seems to handle WMV files better and is good for crappy AVI files too. Keep quicktime as the default player and use VLC for anything thats badly behaved
Install flip4mac anyway as it helps for stuff like movies viewed through your browser