The latest beta of Windows 7 blocks the usage of third party video decoders from Windows Media Player and MCE. Albain, a DirectShow (ffdshow) developer explains in his own words what he encountered when he was testing the multimedia pipeline of Windows 7 in the latest build 7057.
"Microsoft has locked mpeg4 and h264 codecs into Media Foundation, the replacer of DirectShow (even if directshow is still supported).There is no way to override those codecs, even if you develop a Media Foundation version of your decoder, because Microsoft maintains a list of preferred codecs (their codecs) into the registry (HKEY_CLASSES_ROOT and HKLM \MediaFoundation\Transforms\Preferred), and this registry key cannot be modified, even in admin mode. Only TrustedInstaller user can modify it, which is FYI the user that protects system files, and it cannot be used. Microsoft brought those new codecs, but blocked the possibility to use alternate codecs in their applications."
Windows Media Center and Windows Media Player users might need to worry whether things would still be the same in Windows 7 RC or whether it might change, even though there are alternative players options.
The E7 team revealed last month that in addition to built in DivX support, Windows Media Player will also natively support the .mov files.