3
That just shows webserver usage doesn't it? You can run Apache on a Windows server which is done quite a lot, no doubting apache is the most popular though. The fact that getting your hands on a copy of Microsofts IIS 6 webserver (legally) will cost you a minimum of several hundred dollars for the functionally restricted Web Server edition (full blown Windows 2003 Server with IIS 6 will cost you well over $1000), and apache is free of charge, it makes it a pretty clear choice for most.