The NSA probably has the cryptographic keys (which they could easilly get with a FISA court order) for signing Microsoft Windows Updates, kernel drivers and so on, so I wouldn't be surprised if that is their main attack vector for Windows.

There are massive benefits for a State Surveillance and Electronic Espionage entity based in the country of the headquarters of the company that makes the OS and which has special Laws and special Surveillance Courts with secret court orders to let them get their hands directly on the data itself (if hosted on "the cloud") or the official digital keys for pushing whatever they want into computers running that OS.

Windows, iOS and MacOS should be treated by default as thoroughly compromised by the NSA, as should be any cloud hosting in the US or applications from companies based there.

This is not just an American thing: would you really trust a Russian OS or Chinese Data Hosting provider?