Howtoforge posted a tutorial about installing Nagios on Debian Lenny

Nagios is a monitoring solution for complex IT infrastructures, Nagios is easy to implement and can be extended by custom-modules, called plugins. In this howto I explain howto install Nagios on a Debian Lenny host and make the configuration for it. Furthermore we are going to install a second Debian machine which we monitor with remote and local plugins.

  Installing Nagios On Debian Lenny And Monitoring A Debian Lenny Server