http://www.superk.org/index.php/Nagios_&_Windows
Performance Monitor:
Windows NT and better operating systems provided a tool for monitoring many aspects of the Windows operating system called Performance Monitor. Within Performance Monitor there is a wealth of monitoring functions available to track and graph. All of these monitoring functions are accessible through Nagios as well which makes Nagios a fantastic way of keeping track of all your Windows systems. The COUNTER variable in the check_nt command will let us connect to the Windows Performance Monitor and monitor a specific Performance Monitor function.