When XPS Viewer is installed, it adds filters to the system to view and render .xps files. No time was put into making a graphical interface for XPS... so there is no "XPS-viewer". All all the XPS viewer does does is call IE (default browser), passing it the file name.
The problem happens when a second browser is installed and takes the default browser setting. Then "XPS-viewer" passes it to the new browser which does not handle it.
So, change the default application from "XPS Viewer" to Internet Explorer. That is the expected behavior and corrects the problem.
File Properties -> General Tab ->"Opens With", Change button. Set the application to Internet Explorer and it will open the XPS files.