I’m not sure a firewall is your issue. If it is, all you need to do is make sure the windows firewall is set to allow remote connections. You can do that by going into the start menu, then windows firewall, and allow a program through the firewall. Click on change settings(or something similar, sorry-Im doing this off the top of my head), and put a check in the box that says allow remote desktop and remote assistance. Also, you need to go to start, right click computer, then properties. Click on remote settings. Make sure allow remote assistance is checked and make sure if you have an XP computer you check the circle below that says allow connections from computers running any version of windows. If you have Vista or 7 then check the one below. Hope this helps!