From the windows 10 EULA.
"Finally, we will access, disclose and preserve personal data, including your content (such as the content of your emails, other private communications or files in private folders), when we have a good faith belief that doing so is necessary."
"Rather than residing as a static software program on your device, key components of Windows are cloud-based. … In order to provide this computing experience, we collect data about you, your device, and the way you use Windows."
Yikes.