Here is another source that backs up the same claims As we all know Windows XP went on to become the world s most popular operating system by some margin Microsoft has never released any exact figures but my educated guess is that Microsoft has sold over 1 billion Windows XP licenses over the last 13 years However I do concede that some sources say it is only the second most popular OS such as this And this states only 800 million users Perhaps we can compromise on different wording talk