How do I find out why Windows Media Center won't play a movie?

"This is happening cause you have different directshow codecs for X32 and X64, media player will fire up with the x32 version when you click it from the desktop.

However, Windows Media Center runs in X64 mode only, and can not use any x32 codecs. graphstudio has an x64 edition, you could drag your avi onto it to see which codecs it uses to render the video. If a graph can not be rendered then you will need to re-prioritize or install new codecs.

Shark comes with a reasonably good x64 codec pack.

Personally, I would stay clear of X64 media centers, they are not worth the effort I still can not get quicktime streaming to work on x64."

I'd try installing the Combined Community Codec Pack, including the Media Player Classic setup, and trying the file in both the current WMP and Classic.