Do you mean when you stream an Excel or Word file to the browser ?
If your application have a registered mime type or even just an associated
extension, it should load automatically your local application with it
(after the usual open/save as dialog)...
what about a activex? I have an application, user can download it and run
it, but I want to make the launch seamless, when user go to my webpage and
pick some settings, then the application can be launched there(maybe auto
download and install if the app is not installed yet).