Checking services on Windows is a complicated process and I tried answering the question as simple as possible because it was my understanding that others new the additional steps. Let me clear up as much as I can with what I did.
Windows AD Machine Instructions
1 - Install NSClient++ on the Windows AD server.
2 - Installed the necessary plugins for check_ad in the NSClient++ directory
3 - Configured NSC.ini with:
command[check_adir]=C:\Program Files\NSClient++\plugins\check_ad -H $HOSTNAME$ --dc
4 - Ensured that dcdiag runs on the Windows machine being checked
Nagios Server (assuming Nagios is already functional and running)
1 - Install the NRPE plugin
2 - Edit commands.cfg to include something like:
command_line $USER1$/check_nrpe -H $HOSTADDRESS$ -t 20 -c $ARG1$
3 - Edit Services.cfg to include something like:
service_description Active Directory
4 - Restart the Nagios service
These are just examples taken from my current configuration - these will not work for everyone. However, they do give you a place to start from and hopefully have made the complete picture of how to establish a service check on a Windows machine. The following link is something I used to get myself started. My configs were put together from multiple sources, but this is at least where I learned about NSClient++.
Nagios and NSClient++