well did you tried
service httpd status
(for standard installation)
(or is it httpd start)
(for your non standard installation)
now do a
ps aux|grep apache
you should come across something.
Another reason for your port 80 not responding may be that the network service has not been started.You need the lo (ie localhost ) interface to use apache.
(why are you using nmap though,you could try any browser
just type http://localhost/,provided
that your apache is configured in port 80)