In Explorer double clicking opens .xml in notepad.
Control Panel\Programs\Default Programs\Set Associations shows also that current default action is notepad.
But in Directory Opus it gives error -
This file does not have a program associated with it for performing this action. Please install a program or, if one is already installed, create an association in the Default Programs control panel.
It probably has something to do with Internet Explorer, which I have removed. Also removed .xml from ActiveX plugin.
All file handling and stuff is left to Explorer and Windows.
Try right-clicking an XML file and using the Open With menu to assign Notepad as the default program there. That will usually set up the correct registry settings.
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.xml\UserChoice is responsible for that. If there is no value defined, Directory Opus gets stuck, but only on double-clicking or hitting Enter on file, right-click context menu OPEN (bold, which should indicate default action) works with and without that registry key/value. Explorer seems to have ability to fall back to notepad if there's nothing under that key.
...\User Choice isn't the only key that will work, but it's one of several that will.
Which version of Opus are you using? We made some changes recently which added an extra fallback which Explorer uses and Opus didn't. They should only help with incorrect registry settings, but incorrect registry settings aren't that uncommon so we added it.