This is a great blog post on Unixmen about installing Nagios, an open source network and infrastructure software solution, on Ubuntu Server 14.04LTS. Not only does it cover the install but also a basic configuration for monitoring a networked computer.
If you do not know how to install Ubuntu Server 14.04.1LTS then read the following blog post on HowToForge
If you have not got a spare computer lying around but have one with 4Gb of RAM, make it multi-purpose and install VirtualBox on it . Then install Nagios as virtual machine on Ubuntu Server under Virtualbox.
As with everything open source, the only cost is your time to install and your willingness to explore and learn.