My system used to play AVI videos. I think I used Windows media player. Now, all it does is act like an audio player ... good audio, no video. What can I do?

Download and install klite codec, it will solve your problem
download link below

http://extabit.com/file/27b4gacux0str/

You may need to update your Windows Media Player. However, still a lot of people say that they fail to play AVI files with WMP. You can try to download an AVI player program which can support AVI format. In fact, it would be beneficial for you if you download a video player which can support a lot of common video and audio formats. You can try You could try Leawo Blu-ray Player, a free alternative of Windows Media Player, lets you enjoy common/HD movies and audios in various formats on your computer.

The issue you're describing sounds more like missing codecs

In order to play certain kinds of media on certain players you need to have the right codecs. If you want to use Windows Media Player or Winamp to view movies there is an amazing "codec pack" available to download that will enable you to play just about any format.

The pack I'm referring to is called the "K-lite codec pack" and can be downloaded here:

http://www.free-codecs.com/download/K_lite_codec_pack.htm

There's a bunch of good info there if you're interested in exactly what you're getting.

Personally I prefer a program called VLC. "VLC Media Player" works a whole lot nicer "out of the box" than media player, and doesn't require a codec pack to playback just about any movie file you throw at it. I adore this program and can't recommend it over Windows Media Player enough. It's widely regarded as the best free media player available and can be downloaded here:

http://www.videolan.org/vlc/download-windows.html