if you have dial on demand or dial a coneection automatically ...running on the main PC ... and say email on the client ..it may be trying to check email..... or looking for live updates .. can be many things...or one of the apps you have on the client may be trying to phone home (spy ware)
run an app called adaware that will kill any spyware you have , the programs still work but cant send out your details....
what i would do is install a firewall on the client ..zonealarm , and see what is trying to connect to the net through that....