1 in 4 is possible
It would easily be possible to sell that number - remember that companies buy software as well!
But that to one side, in our own household both my teenagers have HP Laptops with Vista ( soon to be W7) , my wife has an older XP laptop ( soon to be replaced by a W7 Netbook) plus the Desktop PC we have and use as a server as well ( Vista soon to go to W7) that's 4 in our houshold plus I also use W7 on my work laptop.
I have been using W7 on a lenovo and also on an older Toshiba laptop since W7 Beta - rock solid on both , performance has never degraded and so stable, even at Beta, that I couldn't dream of using anything else.
Have looked at Apple laptops - very nice and so expensive without offering anything extra in the way of functionality for me - but I can see why people buy them , if they can afford them.
And just to head off the Linux fans - I also have an older desktop that I use to run various flavours of Linux on - just for fun as I was a Unix Consultant some years ago - good to keep my hand in but boy it does feel like stepping back in time...