Você pode usar o K-Lite Codec Pack de 64 bits .
This is a bundle of 64-bit DirectShow filters that can be used together with 64-bit players, such as Vista Media Center. This pack can co-exist without problems with the regular (32-bit) K-Lite Codec Pack.
EDITAR
Nós estávamos indo para trás. O artigo Componentes Win7x64 2.1.8 afirma:
2 versions of Windows Media Player exist on x64 systems. There is a 32bit version stored in \Program Files (x86)\ and a 64bit version stored in \Program Files\
x64 Windows uses the 32bit player as the system default. The installed components of this release include shortcuts to allow you to change the system's default player from the 32bit to 64bit player.
One other Aspect of Windows Media Player(s) on x64 systems needs to be mentioned. If you associate a particular filetype to Open With WMP while the system default is set to use the 64bit player, that filetype will always open the 64bit player when double-clicked. This occurs even if you change the system default back to using the 32bit player.
De acordo com o acima, você deve instalar os codecs de 32 bits. Eu experimentaria a nova versão beta do K-Lite Codec Pack 32bits, atualizado com os recursos do Win7.
Se ainda estiver com problemas, você pode instalar o Win7x64 Components
acima e ver o que ele faz para lhe dar mais controle.
EDIT2
Para encontrar o codec de um arquivo, solte-o em um dos itens: GSpot ou VLC (Ferramentas / Informação de Mídia).
EDIT3
Eu encontrei este thread , que recomenda usar o Utilitário do decodificador do Media Center da Vis :
This utility will show you the current default decoders, let you select an alternative from a list of installed decoders and by pressing Set Default change the defaults
Outros aconselham o uso do nVIDIA PureVideo Decoder ou da desinstalação do FFDSHOW.