If the file association is set correctly and only the icon needs to be changed, you can use something like FileTypesMan to accomplish this.
Select the file type/extension you want to edit from the top pane:
Next edit the selected file type (via right-click/context menu or F2) and ensure the Default Icon field is set to properly point to Notepad++'s executable (or whichever file contains the icons; perhaps a .DLL):
Exit FileTypesMan, log off and log back in or reboot as required, then test your changes
Of course you can do all this from the registry as well, but this utility just makes it easier (and likely safer too). :)
Regarding removal of the Office XML Editor, I didn't find a separate entry under Add/Remove Programs for it, neither could I spot it when I chose to modify the Office 2010 installation and Add/Remove Features. Perhaps I somehow missed it. Anyway, no harm in it being there as long as it's no longer associated in any way with the .XML extension, which is what you want.)
Edit: The Notepad++ File Association utility might also be able to help, otherwise I would recommend trying again via the program itself, or reinstalling so that the installer can set all default file associations properly.