Did you check to see if apache is already running? (Sounds too obvious, but the obvious problems are the ones that usually take me the longest to figure out!)
Go to http://localhost
in your browser, try "ps ax" and look for apache or httpd.
If none of those find anything, try (if you have nmap installed) "nmap -v -p 80 localhost" and see if it finds an open port there.
Barring that, go to your httpd.conf file, and set it to run on another port (say, 81) and try to start it (just to see, I wouldn't leave it there).