Vista and above has this notion of "network location"s for this very purpose. The idea is that you tell it you are at a home and it will allow other devices with an IP address on your local network to talk to your machine.
Follow the instructions here to set/change a network location for the firewall.
If you did not allow Apache to operate on either network locations, you can change it by following the steps here.
You can of course add advanced rules to Windows Firewall to allow certain IPs/ranges etc (particularly using netsh advfirewall firewall for command line or via the "Advanced Settings" option in the Windows Firewall control panel applet which is a short-cut to something someone else pointed you to, "wf.msc"), but I believe that is beyond the scope/interest of your question.
Though these are Windows 7 links, it didn't really change much since Vista.