The 11/6/2007 edition of PC Magazine published stats from W3Counter based on 33 million visits at over 5,000 websites. The market share for operating systems are as follows:
The surprise for me was Linux. I have stats from 10 years ago and Linux had a greater market share back then. I would debate the current stat as there are many good distros of Linux out there now. I've tried Fedora Core, Ubuntu, and Mandriva and these were very good. Yeah, they have a few problems but they don't cost several hundred dollars like XP and Vista.