One of the common reasons why Windows Media Player cannot play AVI video files is that the player does not support the codec that was used to compress the file. To create video files, programs are used to compress the file to make it smaller and for media players to play the file, it needs to have the necessary decompressing program called a "codec" to be able to read the file.
AVI stands for Audio Video Interleave and it is an audio and video container format introduced by Microsoft in 1992. Certain programs, such as DiVX, Xvid and MPEG-4, use codecs to compress AVI files that Windows Media Player does not support. To be able for Windows Media Player to play AVI files, the necessary codecs must be downloaded and installed on the computer.