Problems with Hub

We run on Win2000 Server built on NT4.0
runs through a hub, and connects to internet via router/modem from local cable/internet company.
I am having problems trouble shooting. The problem is slowdown, and when on the internet dropoffs. The cable/internet company has put in new cables, and says that the problem lies in our system. How do I figure out where in our system the problem is. All the computers have the problem. Not sure if it a setting on the server or a bad hub..etc. Help! Oh and if my hub is bad…do you have suggestions for a new one?