An H.264 file is a high-definition video. This format is often incompatible with native video players such as Windows Media Player. If you want to play an H.264 using Windows Media Player, you'll need to download a supplement codec to allow your player to recognize and play a format outside of the standard video types it typically plays.
Download and install some supplement codec software for Windows Media Player such as Media Player Codec Pack (see Resources).
Click "Start" in Windows and select the folder in which your H.264 file is located, such as My Videos or My Downloads.
Right-click on the video and select "Open With." Choose "Windows Media Player" from the available options to begin playing your video.