Check to make sure the firewall on the computer running Apache allows (a) Apache to be accessed and (b) its default port 80 to be accessed. If you have more than one firewall running, check them all. Note that many "internet security" products also need to allow access to Apache and port 80, even if they do not call themselves a "firewall".
For example, you need to open things up in Windows Firewall, AND in ZoneAlarm, AND in McAfee Internet Security.
And by "seems to function perfectly" I guess you mean that when you launch Apache you can launch a web browser on that same system and access "http://localhost:80" to get the local Apache "installation success" page, right?