You can use K-Lite Codec Pack 64-bit.
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.
We were going about it backwards. The article Win7x64 Components 2.1.8 states:
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
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.
According to the above, you should actually install the 32-bits codecs.
I would try the new beta version of K-Lite Codec Pack 32bits, updated with Win7 features.
If still having troubles, you might install the above
Win7x64 Components and see what it does in giving you more control.
To find the codec of a file, drop it into one of: GSpot or VLC (Tools / Media Information).
I found this thread, which recommends using the Vista Media Center Decoder Utility:
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
Others counsel the use of nVIDIA PureVideo Decoder or uninstalling FFDSHOW.