what version of AIM are you using, I had a similar problem when i last formated my computer and i went to dl the latest version of AIM, for some reason it wouldnt let me sign in kept giving me error messages, so i would do most have been suggesing, and try trillian or GAIM
or
Goto
http://www.oldversion.com/program.php?n=aim and try dling an old version of AIM
Let me know how it works out
-Big Mike