can't start Apache on Mac OS X--no listening sockets available?

[7stud]

I'm trying to get Apache set up on my system so I can use mod_python.
I installed Apache 2.2.4 according to the following instructions:

http://switch.richard5.net/isp-in-a-box-v2/installing-apache-on-mac-os-x/#comment-30704

and everything seemed to install correctly, but I can't start Apache.
I typed in the following command:

$ sudo /Library/Apache2/bin/apachectl start
Password:

and I got this error message:

httpd: Could not reliably determine the server's fully qualified
domain name, using tms-computer.local for ServerName
(48)Address already in use: make_sock: could not bind to address
0.0.0.0:80
no listening sockets available, shutting down
Unable to open logs

Any ideas?