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