It's been more than a year since I used the official AIM client, so I might be off here and there, but here goes...
First, you need to reset your default IM application to something other than AIM. Open up the Add/Remove Programs item and select the "Set Program Access and Defaults" tab. Change the default IM application to something else. That should, I believe, prevent AIM from opening up with Outlook Express.
The welcome page you're referring to is the "AIM Today" window, I think. I'm pretty sure there's a way to turn it off in the AIM options/preferences dialog. Try looking for something like "AIM Today" once again. As I said, I don't have AIM installed, so I can't really check.
Finally, if you prefer an older version of AIM to the new one, you can pick up a copy of the installer for just about any version here, starting from AIM 1.0
http://www.oldversion.com/program.php?n=aim