[X-Newbies] AIM hyperlink preferences?

[Steven Rogers]

I have a web page with a link in it like

<a href="aim:goIM?screenname=blah">chat</a>

the operative part of that is "aim:goIM", just in case it gets turned  
into a real link. I expected this to launch AIM or iChat, but it  
launches Adium in Safari, Firefox, and Mozilla. It seems like this is  
some kind of system preference, but I can't find it anywhere - any  
suggestions? (OS X 10.4.1)

Thanks,
Steve