Thanks to Swordmaker for the ping!!
Is it 100,000,000 "downloads" (which means nothing), or 100,000,000 "installs" (which counts re-installs and installs followed by roll-backs to 7 or 8.1), or 100,000,000 actual machines in use (which they... DO NOT MENTION)?
Microsoft has a history of brazenly lying about their sales, so the question is meaningful to ask.
With Vista, they counted as "Vista sales":
Might be legit, in use installations. Might not. I'll wait for the figures from the outfits that sample the web and tell how many people are actually using Windows 10 on the internet (virtually no one uses Win10 as a non-connected system). I'd like to see Microsoft make their goal of a billion devices. But I want to see them do it honestly this time.