Launch an app with an html link

I'm guessing there's no easy way to do this (judging from the utter lack of references I've found online), but I have a linik on a web page that takes viewers to a site to download a chat client. However, what I want to do is launch that chat client when the user clicks on it. (since they all pretty much have it by now). So, what would happen is if they want to chat with us, they click the link and it opens their chat client (we have already configured their chat clients to have a contact set up to reach us).

Any simple way to do this with HTML?

Thanks!

nope, no way at all actually without having code that "register" your application with IE so that clicking specially formatted link would trigger your application. for example, if you install eMule program you can click such link:
ed2k://|file|Final%20Fantasy%208%20...RHTV5NJOXRMM6|/
and it would open eMule and search for that file. how? eMule has registered the "ed2k" with itself, same way as clicking "http://..." link would open your default browser. dunno about exact code but I believe this is the only way..... which is far from being simple.