Maybe you could leverage some kind of a tool which inspects the traffic and catalogs the bandwidth usage. There are a lot of them out there, though, and choosing the right one might be tricky.

Ideally you'd want one which just totaled up all the data flowing to given addresses, and give you a usage report showing which network addresses were responsible for the bandwidth usage, correlated perhaps to which apps/processes on the PC were doing the using. Maybe along with some DNS or Whois lookups to tell you in plain english which sites these processes are connecting to. Most network monitoring tools (like this one I think can do this as a subset of their full suite, but the overall product is designed to do so much more than that, that it'd probably make it not very easy to use for that one simple task.

Anyone recommend of a small, simple utility to do that job?