php module for apache not working

[H.S.Rai]

On clicking a php file, through web server, instead of processing it,
it ask for download. apache2 and libapache2-mod-php5
apache2-mpm-prefork, php5 are installed.

I am using Ubuntu 7.04, Feisty Fawn.

However, on restarting apache2, it gave following error:

 sudo /etc/init.d/apache2 restart
Password:
 * Forcing reload of web server (apache2)...
         apache2: Could not reliably determine the server's fully
qualified domain name, using 127.0.1.1 for ServerName
httpd (no pid file) not running
apache2: Could not reliably determine the server's fully qualified
domain name, using 127.0.1.1 for ServerName
(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

But webserver is running, and I am able to access static HTML pages.

What could be problem?

-- 
H.S.Rai