This is my section on Nagios, currently I am still learning and exploring the world of Nagios. So this is the start of my Nagios knowledgebase, but we'll see how this goes.
By way of an introduction though, Nagios is a monitoring product. It is an open source product but can also be purchased as a commercial version or you can purchase support. See Nagios - The Industry Standard in IT Infrastructure Monitoring for more details. You will see many references or variations of NRPE, which stands for Nagios Remote Plugin Executor.
I think NSClient++ | The next generation monitoring agent works well when monitoring Windows machines. A cross platform alternative is JNRPE – Welcome to the JNRPE package which is a Java based NRPE alternative.
The official documentation is at Nagios Core Documentation - Table Of Contents which includes how to monitor different types of client.
Download the server installation package, which is a jar and execute the jar as follows:
java -jar jnrpe-server-install-2.0.5.jar
This launches the GUI based installer. Then you can start the Windows Service called "JNRPE Server".