ftp://ftp.3com.com/pub/3c510/3c510ug.pdf, page 14:
Enter "Service Port" 6881, "Server IP" is the IP of the computer that runs bittornado, check "Enable". You have to repeat these steps if you want to open up more ports. You only have to open as much ports as you want run instances of bittornado at the same time.
You also may need to assign a static IP to your computer, because when your IP changes (e.g. reboots), the ports are still forwarded to the old IP.
To disable the XP firewall go the windows control panel, network connections. Then right click on your internet connection (the connection to your router) and choose properties. Now go to the Advanced tab and uncheck "Protect my computer and network...".