Internet goes on and off at the PCs of my work place.

The network there works with a domain server.

They use a combinations of hubs and routers to connect with each other,
some of them with windows 2000, some with XP, the server is 2000.

The problem is that a couple of days ago Internet service started to go
off on all the PCs of the network, then it would go back on.
This continued for a couple of days, until I contacted the ISP, blaming
them from the problem.
They said there are OK, so the problem is ours.
What could it be? I know very little of networking, any help greatly
appreciated.