In Windows you'll use the command prompt or MSDOS with the netstat command.
Type netstat /? for more information about the command.
The basic home router doesn't really block your ports like you would think, it just hide your computer and it keeps the bad guys from detecting your computer when they run a scan on your public IP. Just like router protects you from the outside world, it does not protect you from the inside network, therefore anyone inside your network can run a scan and detect what ports are open,listening or close on your computer. Tools like nessus will detect ports and scan for vulnerability on your computer. It's commonly used by security experts and network administrators to detect flaws in their network before some other bad guy do.
With no router or firewall or any application, all your ports are in listening mode, they are only open when they need to be used and they are only blocked when a firewall or a software restricts them.
Accepted or Open: The host sent a reply indicating that a service is listening on the port.
Denied or Closed: The host sent a reply indicating that connections will be denied to the port.
Dropped or Blocked: There was no reply from the host.
So in order to use a service on the inside of your network, the router must know which computer to forward the request to. Therefore you use port forwarding,
www.portforward.com will give you instructions for the BitTorrent application and the model of your router.
Things you should be aware of, most home virus detection software are now build with simple firewall functions like blocking ports that are used by many Trojan horses and worms. Therefore you have to take into consideration that some legitimate ports may be block by your virus detection software and you'll need to also configure it to open that port. Disabling it is not a good idea. Open the ports you need and deny the rest, that's the best idea.
