All systems? Wrong again.
AS/400s, mainframes, Macs, RISC processors, etc., aren't even capable of running Windows.
Those are mostly servers, which are typically outnumbered by at least 10 to 1 by desktops, and server sales are currently 70% Windows anyway, so you're only talking about .3 of the .1 maketshare servers even have in the tally of all computers. I said ~90% which is still probably about right, will you settle for 88%?