The installer of UltraEdit adds UltraEdit as HTML source viewer in Internet Explorer on making a complete
installation or on making a custom
installation if the installation option View Source Editor
is not unchecked during the installation.
The real problem is that Microsoft has not registered Notepad as source viewer and simply uses Notepad, if no source viewer is registered. So the first program, which registers itself as source viewer for HTML files is automatically used by IE as source viewer. HTML editors have to add itself to the list of source viewers, because no user can do it manually, except the user is an expert and knows how and where to modify the Windows registry. There is no option for the user to configure anywhere with the Internet Options of Internet Explorer the application which should be used for viewing the source of a displayed page.
See also UltraEdit takes over Windows Notepad as source viewer in IE
Did the installer of UltraEdit not asked you about setting UltraEdit as View Source Editor for web pages displayed by Internet Explorer?Update:
The installer option View Source Editor
does not exist anymore since UE v18.20. Therefore all installers of UltraEdit v18.20 and later never register UltraEdit as source viewer for pages displayed in Internet Explorer.