Using just HTML (or JavaScript) it's impossible, at least if you don't have some sort of listener on the client side. But there's a nice and elegant solution proposed by Sun, with Java Web Start. It is composed by a client java application "java web start" (let's call JWS) that interacts with the browser registering a mime type "jnlp", and when the browser opens a jnlp (with a click on an image or an href for example) its stream it passes the control to JWS that reading it learns from where to download/update the appplication to start. So it handles much more than just starting local apps (providing a good way to handle software distribution and updates).

I strongly recommend you to give this a look, and to not waste time re-inventing the same thing.

Here I'm working with a bad-designed (hoping my chief won't read this...) legacy app that listens on port 80 on the client side and gets requests from the browser (using href="localhost?app=xyz") to launch applications on the client disk. This is not nice because requires a service (web) listening on the client all the time with the overhead of the jvm (and wasting lot memory). Also this does not solve software distribution, updates, and implies also an administration of the launching tool at the client and a the server side.