Amazon.com - Today's most popular operating systems. Mac OS X is #1, Windows is #2 and #3, SuSE Linux is #4.
They aren't Dell, so you aren't seeing Dell's much higher volume of Windows PC's. Most copies of Windows XP are sold with the hardware, not separately sold, as with the annual upgrades to Mac OS X. Amazon is clearly doing better on the Apple side of things, both hardware and software, than some vendors.
Another problem with these stats, not evident here, is that many copies of Linux are running on hardware sold with a Windows license. The three desktops that I am switching between right now are like that.
And yet another problem with such stats is that there are many brands of Linux sold or downloaded, and that each copy of a Linux distribution is licensed to be installed as many times as you like. So the original sales of a single Linux distribution are a much smaller percentage of the installed base than with Mac OS X or Windows.