Hi waleranbigod,
It is a most common problem that you are facing because either you havent downloaded and installed the correct Codec pack software or it might have been corrupted if already installed or Windows Media Player isn't properly associating the correct codec on your system in a way that makes playback possible. So the first step is to install the correct codec pack software in your computer. I usually use a software called GSpot that checks the avi required file in the correct manner. So
download and install GSpot and check whether you face the same problem or not.
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