IF you're using apache 2.2.x or 2.0.x it should be: C:\Program Files\Apache Software Foundation\Apache 2.x.x\conf\http.conf
If you're using 1.3.x it's the same, just instead of Apache 2 it's Apache 1.x.x.
You could always modify one of the httpd.conf files (say, remove the server_admin email) and then restart the server. If it successfully restarts, you know that it's the one that was not edited; however, if it doesn't restart, you know it's the one you edited 😉