I have downloaded XULRunner from http://releases.mozilla.org/pub/mozilla.org/xulrunner/releases/7.0/runtimes/xulrunner-7.0.en-US.win32.zip
Unpacked this zip into fol
Support for "registering" XULRunner has been removed in XULRunner 5.0: https://bugzilla.mozilla.org/show_bug.cgi?id=642795 (it actually wasn't exactly useful anyway). So by unpacking XULRunner you already installed it. Now you can go find or create a XULRunner application and run it with xulrunner.exe --app path\to\app\application.ini
.