Sounds like you are typing the user and pass incorrectly:
- Have your previous employer fwd your email to your new account. (they may have terminated the account)
- Change your AIM account info (new email and pass) by following the security questions (if possible)
- Find the correct info that was in the KEYCHAIN on your old PowerBook. (if you still have it) Go to: Finder>Applications>Utilities> Keychain Access.App
Adium is a good IM application to use for chat.
Best.