how to start up a desktop application in client side

Basically it's not possible to achieve unless an application registers a protocol that will trigger it. If it does that all you need to do is to provide a link using this protocol

yourcustomapp://some.parameters

Another way the 3rd party app can integrate with the browser is if it hooks to it as a plugin. This is how flash apps work etc.

If the app you are trying to launch does not support something like that it's going to be close to impossible to achieve what you want.

The browser sandbox prohibits you from executing local resources, for good reason - to thwart a website destroying your box with malicious code. I've been researching the same functionality.

The only solution I've found is to build an extension in Mozilla Firefox which can launch your app. Extensions live outside the sandbox so they can execute local resources. See this page for how to do that. You may be able to do it cross-browser using crossrider, though I haven't had success with that yet.

You could alternatively build a thick client populated from a web service, and launched from the browser through an extension as mentioned above. This is what I'm doing to get around the sandbox. I'm using local XUL for this.

See my question for additional discussion.