Windows had never made it into "essentially .NET". Longhorn was going to be the first one, but it was scraped and Vista is essentially Win32/Win64 APIs with pre-installed .NET framework. I doubt there's anything in Windows client that is written in .NET - server is a different story though.
I did not say that Windows was written in .NET. The parent comment was "essentially .NET for many purposes." Among those purposes would be development of applications and system automation.