yo 8-)
first, I must state that 80% of the people just get the "apache is installed" page correctly uopn first try ;-)
I'm a bit surprised by the error... I think I've to look into your httpd.conf file
Also, I would like HOW you launch Apache.
Do you want my first guess ? 1) you installed by default, ie as a service ; 2) Apache is thus already start"ed ; at least the socket is in use, and on port 8080 which is the port to use in your browser ; and 3) you try to launch Apache a second time ("in console mode" for instance)
try
http://localhost:8080