How to install Nagios on CentOS?

Nagios is a powerful monitoring system that enables organizations to identify and resolve IT infrastructure problems before they affect critical business processes.

Nagios monitors your entire IT infrastructure to ensure systems, applications, services, and business processes are functioning properly. In the event of a failure, Nagios can alert technical staff of the problem, allowing them to begin remediation processes before outages affect business processes, end-users, or customers. With Nagios you’ll never be left having to explain why a unseen infrastructure outage hurt your organization’s bottom line.

This howto will help in installation and configuration of Nagios on CentOS, I am assuming that you have installed CentOS 5 with all necessary packages but minimal installation is preferred.