The default port of LW is 6346, so both (listening and manual port forward) should be the same.
As for the firewall, it can have an effect if it is not configured properly.
Right-click you Network Connection, which should be the Local Area Connection, then go to Properties and in the General Tab you click on the Internet Protocol (TCP/IP) and then Properties. There you should check whether the IP and Submask is the same as the ones you have, if they aren't type in the ones you have.
Then go to Advanced>Settings and add the listening port 6346 as TCP once and then as UDP once and then click okay. Could be that you have to reconnect to the internet. See if that changes something.
Oh, almost forgot,....after you changed the manual port and the listening port to the same number, close LW and then follow the steps above.
