wmp can't play the avi files because they're probably encoded using an unsupported codec - maybe h264 or whatever.
not all avi files are the same - there's quite a lot of confusion regarding video files, there isn't a single avi 'format' - basically avi is just a 'container', and the video can be encoded in many different ways. you need a suitable codec installed on your system to translate the zeroes and ones into a video stream.
if you really need wmp (or media center) to play the file, you'll need a codec pack - shark007's pack
is probably the best there is.
software such as vlc has all the codecs 'built-in'.