Do you know who your ISP is? Are you living in student accommodation or something, where everyone connects through the building's network (I'm guessing this as you don't have a modem or router)?
If so, you might have to talk to whoever's in charge of the internet service & find out the policy on p2p use.

You could try changing to a higher port (try one in the range of 49152 - 65535). If you do change the listening port, remember to also change port numbers allowed through the XP firewall