Slight problem with the NSIS installer writing the entries for WMP. Found this out myself trying to do the same. The installer needs to write a "MediaType.Icon" entry for each exension in "HKLM\SOFTWARE\Microsoft\MediaPlayer\Player\Extensions". Even if the entry is blank that's fine, it just needs to exist. The problem occurs when you open WMP and goto the File Types association menu, you will not be able to turn on/off the extensions added by the installer. Once they are turned off they will permanently stay off. The "MediaType.Icon" entry is somehow required in order to set the associations from within WMP's menu.