Sounds like you migth get some benefit from the Network Traffic Analyser product, of which there is a free version.
NTA uses Cisco's Netflow system to provide details on flows of traffic between endpoints. Netflow takes a little configuring, but there is a Netflow Configurator in the Engineer's Toolset (not sure if there it is in the free version or not).
I think most Cisco devices (switches and routers) support Netflow, but you may want to check first.
Of course, there is some argument here that what you need to do is have a web proxy (or web proxies, depending on your setup) that can report on the internet usage of individual users if your main problems are internet download-related!
There is a Netflow configurator in the free version as well.