The accuracy of these figures is unknown. StatCounter puts Windows 7's marketshare at 17.68 percent, NetMarketShare gives a figure of 21.67 percent and USGDAP says it's 18.9 percent. Each firm uses a different methodology.
Of course the lion's share of the rest of the Windows user base is now on one of the versions of Windows 10 available. The third placed OS in the three surveys was Windows 8.X with as much as 5 per cent of the user base, depending upon source. All sources seem to agree - Windows XP usage is below 1 per cent now.
At this stage of a review of the reporting sources it is always useful to remember they are all online counting methods. Thus if an organisation is running a number of Windows XP PCs, for example, that don't access sites monitored by the likes of StatCounter, for example, they won't be counted.