Make sure that a media player is installed on your computer. For Windows computer, make sure that Windows Media Player is installed completely. There are instances that Windows Media Player looks installed on the computer but has to be configured in order to use it. If so, just open Windows Media Player and follow the steps to configure it.
If no media player is installed on your computer you can download free media player application online like VLC. Just make sure to check if the application supports AVI video format before downloading and installing. Once installed the computer will automatically set it as the default media player so when you open a video file it will automatically open and play the video.