First of all, I can't promise it will work in your environment. That is one of the reasons I did not put it on the Nagios Exchange, as I was expecting too many people with issues...
It works over here with UAC 95 % of the time. But if you have some spare time, you could give it a try:
You could start with trying to get the installation script 'naf_install_nscp_0.4.1.101.ps1' to work on a test server, which has no NSClient installed. Put the 32bit and 64bit msi files in C:\Nagios . The script was made for 0.4.1.101, so choose this version, or you will have to edit the script to make it work for 0.4.1.105. Then try execute the naf_install_nscp_0.4.1.101.ps1 as administrator. Make sure executionpolicy is set to remotesigned. Once you get it to work there, continue to making it work from the Windows gateway server you will use.
The NSClient service on the gateway server should runs with an account that has admin permissions on the other Windows Servers. Then before you start with the Nagios side configuration, install Powergui on this gateway server, a Powershell IDE, with which you can easily troubleshoot the scripts and see what's going wrong.
You should execute the naf_request_install_nscp_0.4.1.101.ps1 on the Windows gateway in PowerGUI. Add the host where you want it to install on as a parameter.
If you get to this point, let me know, then we'll jump to the Nagios part. If you don't get the above to work, there is no point going further..
Grtz
Willem