Yip, If you're leaching someone elses bandwidth then you have no say on what ports are or are not open.
If you have just a cable modem then it should be straight out to the internet from your pc...Check which port bit torrent is trying to connect through and change it to something else as the standard port of 6881 is normally blocked by ISP's.
Also check that your windows firewall is open for the new port...Normally when you reopen your bittorrent client windows firewall will ask if you want to let it connect and this will open the port.
Good luck.