If both pc's are behind the router then no port-forwarding is necessary. That is only necessary if you connect to someone on the internet. Also since most routers also have a DHCP server built-in then both pc's would get a different ip adress automatically. This adress can change every other day or even after some hours (usually with wireless connections) if the pcs have been turned of for some time. The join-er should enter the IP-address of the host-er. In windows 7 you can find the ip-address of the computer (do this on the hosting computer) the following way:
Go to "Network and Sharing Center" (right-click on network icon in taskbar),
Locate the network connection to the LAN in the list of active networks. On the right side there should be the name of the network connection as some sort of url-like link. Click it. (a status window for the network card should show)
Click the Details Button,
Locate the 4-part code behind "IPv4 Address". That is the IP-address of this PC in the LAN. (it should be something like 10.x.y.z or 172.16-32.y.z or 192.168.y.z)
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