Change Preferred Direct Show Decoding Filters in Windows 7
When Windows 7 was in Beta, it has been discovered that Windows 7 Media Player and Windows Media Center will block the usage of third party video codecs. Fortunately later Microsoft has confirmed that the final release of Windows 7 does not restrict the use of third-party codecs. However if you try to install K-Lite Codec Pack, you will notice that the third party drivers are not being used and the only way to change it is by editing the registry.
Fortunately there is a free utility called Win7DSFilterTweaker developed by CodecGuide which allows you to configure your preferred DirectShow decoders with just a few mouse clicks. It is quick, easy, and changes can always be undone. If you for example prefer to use ffdshow or CoreAVC for decoding H.264 video then you can do so by using this tool. This tool is particularly useful for the Microsoft players, namely Windows Media Player and Media Center. Certain advanced third party players, such as Media Player Classic, are already capable of using third party filters without using this tool.
The changes made by Win7DSFilterTweaker is permanent and takes effect immediately without reboot. To restore back the original Microsoft codecs, you will have to run Win7DSFilterTweaker again, and select Microsoft from the list.