This short tutorial describes how to set Notepad++ as default editor for .xml files. Simply register the extension in the app's preferences.

A lot of ASP applications produce their output based on the contents of one or more XML files. XSLT documents transform the browser-neutral contents of those XML files into browser-specific HTML ...

Keep in mind that you could also create XML files that absolutely ruin the end image; they’re not always guaranteed to give the best results.