The w3counter.com site published several statistics based on 33 milion unique visits to 5664 sites. The statistics refer to the operating system, web browser, country and screen resolution of the visitor. The king of kings of operating systems is Windows XP with 83,48%, followed by Windows 2000, Max OS X and Windows Vista. Linux and Windows 98 have both 1.34% percent. In terms of web broswers, the leader is IE 6 with 47.01%, followed IE7 with 19.19% and Firefox 2.0 with 17.51%.
You can see the whole charts here: http://www.w3counter.com/globalstats.php.