Did you try to re-register the screen name? I don’t know if AIM purges old screen names or not, but it can’t hurt to try. I got locked out of an old Yahoo! login several years ago*, and the only way for me to get it back was to wait until the account was deleted so that I could re-register with the same name.

*(I mistyped the password a few times and then they started wanting me to answer security questions, which I can never remember. Now I know to record my questions and answers in a safe place.)