Ubuntu error with apache: (98)Address already in use

I am getting this error when I try to start Apache in Ubuntu.

 (98)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
 Action 'start' failed.

I have this in my ports.conf

NameVirtualHost *:80
Listen 80

This is my vhost file

<VirtualHost *:80>
          ServerAdmin example@example.com
          ServerName rails.server.com
          # ServerAlias
          DocumentRoot /var/www/sample_app/current/public
          ErrorLog /var/www/sample_app/error.log

          RailsEnv production
        <Directory "/var/www/sample_app/current/public">
          Options Indexes FollowSymLinks MultiViews
          Order allow,deny
          Allow from all
        </Directory>
</VirtualHost>

What am I missing?

It seems port 80 is already taken. Use another port or try netstat (grep the result to select only the row with value 80 in it), ps and kill to see what application occupies the port and shut it down.