As others said your Firewall needs to be configured to accept incoming calls on TCP Port 80.
in win 7+ (easy wizardry way)
- go to windows firewall with advance security
- Inbound Rules -> Action -> New Rule
- select Predefined radio button and then select the last item - World Wide Web Services(HTTP)
- click next and leave the next steps as they are ( allow the connection )
Because outbound traffic(from server to outside world) is allowed by default .it means for example http responses that web server is sending back to outside users and requests
But inbound traffic (originating from outside world to the server) is blocked by default like the user web requests originating from their browser which cannot reach the web server by default and you must open it.
You can also take a closer look at inbound and outbound rules at this page