If you have downloaded videos from websites like YouTube, chances are that it is of FLV (Flash video) format. FLV format is the preferred format for content distribution online.
Windows Media Player does not play .flv files. However, by using a plugin called Play FLV, we can play .flv files on WMP.
Play FLV: Open Flash video files in WMP
Download Play FLV plugin.
Double-click the Play FLV setup file to begin the install process.
Installation is easy, you basically have to click Next a few times.
Play FLV installer will ask you for permission to download (internet connection required) and install FFDshow that is required to play Flash 7 video files. Click Yes.
Once download is complete, click OK to start install of FFDshow.
Once again, you have to click Next a few times to complete the FFDshow installation, as no settings have to be tweaked.
Installation is now complete. Play FLV associates itself with FLV files, so once you install it, double clicking an .flv video file will automatically open in Windows Media Player.