Generally, windows by default doesnot have the Codecs required to decode the video files encoded in formats like Xvid avi, Divx , ASF, MP4 etc. So, you will need a special video player that provides windows with the codecs that can allow it to play such videos.

K-lite codec pack is the best tool to use . Just download it from http://www.free-codecs.com/download/K_lite_codec_pack.htm for free and enjoy watching videos.