You need the correct codec (stands for COmpression/DECompression) in order to watch the movie. AVI movies for the most part are a compressed format, so you need to decompress them. The most common codec used for AVI movies is DivX. Here's a link where you can get codecs. Some like codec packs...some like alternative players like VLC so they can watch any movie (many codecs installed with it). As long as the codecs you need to watch the movie are installed on your computer, Windows Media Player should be able to handle the file. Here's a helpful link for Codecs & Alternative Players

Good Luck