If you haven't done so already run one or more spyware and virus checkers. It is possible that something unpleasant has got into your system and is using your bandwidth for nefarious purposes. Try Adaware, Spybot and the Microsoft spyware checker (all free downloads). AVG antivirus is good and free as well.
Do you have the firewall turned on? At the very least the windows firewall should be on, but there are better (and also free) alternatives such as Zonealarm.