The sample source code download link at the beginning of this MSDN article, Building Plug-ins for Network Access Protection, unfortunately became broken at some point along the way. You can find the source code here: Registry SHV/SHV reference code for NAP.
Disclaimer: that code is not being maintained and has not been updated since the article was written (pre-Windows Vista RTM). That said, the NAP interfaces have only experienced minor changes, and I expect that the code would still work as-is.
Please note that Microsoft published a newer NAP sample with the Windows 7 SDK. Their version has improvements over mine, including in the COM activation settings for the SHV. However, my version supports remediation. Therefore, I recommend that developers reference both versions when planning a new custom implementation.