Same thing used to happen to me until I updated to BitTornado 3.18. It has encryption capability which fights Comcast's attempts to throttle or limit bittorrent activity. I use the default encryption settings and am now able to upload after completing a torrent. Use ports above 50,000. The defualt ports (6881-6889) are now known to be used for BT by ISP's, so they limit them. Just check that Windows Firewall is not blocking you (Control Panel > Windows Firewall > Exceptions). It should have btdownloadgui in the list. Make sure it is checked. I don't think you have to forward your ports because you are not using a router.

http://www.bittornado.com/