We are a small office that has a T-1 line coming from the street. The
configuration is

ISP T-1 line -> Cisco IAD2430 -> Linksys SRW224G4 switch -> RJ45 wall

The Cisco is serving as our DHCP server.

A serious issue we have now is that sometimes any particular PC or MAC
can hog the entire bandwidth of the T-1 line. For example, whenever a
MAC does a system update that downloads some big files, the rest of
the office cannot really get on the internet.

Is there a firewall, router or switch that can prevent this situation?
I have play with the QOS settings in the SRW224 switch, but still it
does not help. And I asked our ISP if they can configure something in
the Cisco IAD device, they say it does not provide any QOS
functionality of this nature.

Please help....