DH uses
www.meebo.com to access Yahoo IM when he is at work since the Army won't let him have any IM service downloaded on his work computer. I think you can sign into other IM accounts as well but only 1 at a time.
No downloading required.
I have never had an issue with Yahoo but have had major issues with AIM when I downloaded it on my last laptop (it kept causing my system to crash). Once I removed AIM the crashing stopped.