The primary installation that comes with Mac OS X is Apache2.
The executable is found in /usr/sbin
$ /usr/sbin/httpd -v
Server version: Apache/2.2.9 (Unix)
Server built: Sep 18 2008 21:54:05
The config file is found in /etc/apache2/httpd.conf
The document root is found at /Library/WebServer/Documents/inde.html.en
This version is started/stopped from the Mac OS X control Panel -> Sharing -> Web Sharing checkbox
Hopefully this helps you dissect out any problems that you might have with multiple installations.