Have you tried using a badge installer? A badge installer is a piece of flash you put in a web page, and when the user clicks it, it installs a given AIR app (and the runtime if necessary). However I believe that if the app is already installed, then clicking the badge installer simply launches the app instead. So the same piece of Flash would install the app for anyone who doesn't have it, and launch the app for people who do.
A tutorial is here.
Edit: I assume you're trying to launch an app that the user has already installed. If you mean to serve an app to the user and launch it without installing, then as pinnaclear says, that's definitely, definitely not possible.