I've recently acquired Windows Vista, or else this might have been easier for me to fix on my own. I have Mozilla set as the default web browser - I checked under "Default Programs" on the control panel and I went to Tools-->Options-->Main--->Check Now (to see if Firefox is default) within the browser itself and both say that it is. However, when I click a link in AIM, IE opens. For awhile Avant Browser opened, but I uninstalled that to see if it would fix the problem, and now IE opens. This is really annoying...how can I fix it?

You can't fix it. Many programs (such as web-links in Outlook Express, for example) are "hard coded" to open links in Internet Explorer only, and will ignore your default browser setting if you changed it to something else. I imagine AIM is another example of this behaviour. For that reason, even though I always use Opera as my browser of choice, I leave the default set to IE. It's pointless doing otherwise if an alternative default is often ignored by some applications.

Hmmm...why would it open Avant then? I know Avant essentially sits on top of IE, but they're not the same thing...