Virus

Respected Sir/Madam,
I have a cyber cafe with Dishnet DSL Connection, I have 8 Machines being Celeron 500Mhz, all having Windows98. The Problem is that whenever I browse www.yahoo.com site the Machine gets stuck up and it works very slowly taking pauses I have Norton2003 installed on every Machine which is latest updated but still I m facing the Problem.
Sir I request you to give me the solution for it atleast tell me whether it is infected with Virus and which (name).
Thank You.
Shailesh.