The reason I don't want to go the browser plugin way is that you need to implement it for various browsers.
Like xpi for firefox, browser helper object for IE.
My target platform is only windows, and I just want the alternative of browser plugin to call a client side programe after instructed when the user is browser the web page.
Is that viable?
The only "viable" solution would be to register a protocol handler system-wide so say addresses starting with "myprotocol://" trigger your app. Users will still need to install your program and your program will only have access to the parameters passed by that protocol handler, so you will have to evaluate on your own if it's worth it.
Also note that some browsers or settings might show a confirmation message before using your new protocol for the first time, so users should be informed on what to do (and warned that this prompt is part of the normal workings of your app).