Broadband connection problem?

I have been using broadband connection for the past six months from BSNL. I experience one problem for past one month i.e. whenever I switch on the modem; bytes get transferred aggressively even if I don’t browse. It continues to get transferred even if I close the browser. Because of this my account usage goes up. So I switch off the modem. I use anti virus software. When I scan the system, it shows no virus found.