If you installed codecs from xiph.org, the installer destroys several Windows interfaces such as the Mediatype dropdown dialog box of Windows Media Player 'Open' menu and the Windows handling and organization of the following filetypes: wav; snd; au; aif; aifc; aiff; wma; mp3
If this is your issue, I have incooperated a fix for it into my codec releases.
If you havent installed the codecs from xiph.org, please ignore this post.