My website is hosted by Microsoft on Windows servers, but having administered many servers (Novell NetWare, OS/2, Unix, Windows NT4 thru Server 2003, Linux), Windows would be my last choice for most server functions.
Management likes Windows servers because they think that the graphical user interface will lower the cost of administrators, but total cost of ownership is greater for Windows servers (greater hardware requirements, less MTTF).
I had heavily used NetWare servers that would run without rebooting for 11 months, yet Windows NT4 nd 2000 servers that were less lightly loaded needed to be rebooted every week or two, or their processes would die or slow due to memory leaks. NetWare and Unix servers, though tricky to set up and tune, would run and run without attention.
Unix servers can execute PHP, which is a powerful server side scripting language. Windows web servers can't run PHP.