AIM offline, Internet online, screen name is valid

I am able to get online just fine but, when it comes to using AIM, MSN or any other chat software, I get refused by the host or I'm told that I'm not onine. The screenname works and I can log in on other computrers In the house

Firewall/content filter running possibly blocking ports?

nothing has been changed

Doesnt make sense that you can not connect unless it was something filtering the ports out. You got me on this one

Ya, and this has happend to me and someone on appleswitch. I dono what to do.