100% Windows.
At least one ALSO has a Mac, and another uses Linux for everything but electronic gaming. However, both are, in fact, electronic gamers and understand that most of the software they want is only available for the PC. Actually, the Mac-user seems to prefer his PC anyway, probably because it's a laptop.
A bigger issue in my group is that two don't have internet access, and one doesn't have a computer.
So, 17/18 have Windows, 1/18 has no PC, and 15/18 have internet access.