Guide To MPEG Playback In Windows Media Player
Any video with the extension MPEG or MPG will either be encoded with MPEG-1 or MPEG-2. DirectX includes an MPEG-1 decoder. MPEG-2 decoders, however, are available through third parties due to licensing costs.
Windows Media Player will not download MPEG decoders for you. So if Windows Media Player says Failed to download codec or your MPEG's do not play you will want to follow the below steps.
Reinstalling DirectX will overwrite quartz.dll and re-register the file in Windows. quartz.dll is the MPEG-1 Decoder that WMP will default to.
Reinstalling your MPEG-2 or DVD decoder would be the next step. Windows Media Player does not include an MPEG-2 decoder. If you can get this error for a few MPEG files, there is a chance they are MPEG-2 video. You can use the Windows XP Video Decoder Checkup Utility to see if you currently have an MPEG-2/DVD decoder installed if you are running Windows XP. From within the program you can then set it was the default MPEG decoder. If you see nothing listed, you would want to reinstall that application in order to reinstall the decoder. WinDVD and PowerDVD are two examples of software that would install MPEG-2 decoders that Windows Media Player can use.
Please see DVD and MPEG-2 Support and Windows XP Video Decoder Checkup Utility for more information about MPEG-2 and DVD Playback in Windows Media Player.
Other Notes: Reportedly Ligos Corporation MPEG Splitter (lmpgspl.ax) has also been a target of broken playback. Locate the location of lmpgspl.ax on your PC and then Unregister it. At that point you would likely want to reinstall DirectX again, just to be sure. You can load your MPEG into GraphEdit (Don't run any .BAT files you download with GraphEdit, they are not needed) or the latest beta of GSpot to see if Ligos' Splitter is being used. In addition you can use either GraphEdit or GSpot to see if you have another third party splitter being used.
Media Advice Archive
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Windows Media Player FAQ
Why does the Player display a visualization instead of the video (or return an error message) when I try to play an .mpg or .mpeg file?
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