After updating the ADMX files in you central store, you might see this error in GPMC:
Encountered an error while parsing.
Expected one of the following possible element(s)
This error is for example seen if you try to open the administrative template node from a Windows 2008 server but your central store is updated to Windows 2008 R2 or Windows 7 ADMX files.
From now on you should only use Windows 2008 R2, Windows 7 or newer to edit your group policy objects.