Let me tell you what my wife and I do. First of all neither one of us uses AIM, although we had AOL, and AIM is probably is installed. One of the things we had to do was to synchronize AIM and the Netscape IM(NIM). The only reason for the use of NIM is to set up Netscape or AOL Webmail. I still have Netscape Webmail set up. The two of us have four profiles set up, and we have shortcuts to access each profile. We don't use Master passwords, so we can access the browser part of Netscape normally. We may or may not have saved the passwords of the mail accounts. So, any Netscape is accessible, for the most part.
Now, I don't understand this part of your post:
I went to sign on to my Netscape homepage, his password was now where my old screen name should have been.
I am interpreting that you are using the Master Password, something which I have found unnecessary, and know nothing about. You can take a look at Edit|Preferences|Privacy|Master Password.
I want to make sure that you are asked for a password as you are trying to get into Netscape, or what. The situation is unfamiliar to me, so some details may be needed. But, I can't see how AIM may be involved.
UserAgent: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.7.5) Gecko/20050302 Firefox/0.9.6