Ok, try downloading Media player classic and install CCC pack, that will read any video file you need.

it depends on which Windows media you have, but you would have to go to file, I believe, or settings and tell WMP what files you want it to play.

Or you can do what Yinnie suggested, or you can always download iTunes, which plays almost everything.

Usually when it doesn't want to play AVIs it's because you need to download a Codec. Search google for 'Windows Media Player AVI Codec' and see what turns up. That's what I did to mine anyway, and it works now.