Windows won the desktop, looong ago before Linux was even born with a simple "law" called Microsoft tax.
But linux is on the servers, on the routers, on the fax machines, on the various NAS devices on the phones, in the new PC bios on the TV's, on the PVR's in the airplanes you fly on the medical equipment in the hospitals, well basically Linux is everywhere.
If you want to go per device Linux would outnumber Windows easy. Today the problem is that more and more things start to happen away from the desktops and that is a real problem Microsoft is trying to address. We will see how good they address it with Mango or Windows 8 in the coming months/years, for now they losing like a blood red Ferrari with 4 flat tires at the F1 semi finals.