Microsoft Windows is the world's most widely used operating system on both servers and desktops.
Microsoft's first version of Windows was released as a text mode GUI window system in 1985, running on top of MS-DOS. It took several years and many versions to catch up with features available on platforms such as the Macintosh, OS/2 and X11, and while there were other systems that were technically better than Windows for several years, the great Microsoft marketing machine diminished the relevance of other platforms by fair means or foul, until the mid-90s where Windows was the only operating system that was available to most people.
Windows has come to symbolise all that the open source world hate about proprietary and commercial software. However, Linux advocacy is not about "replacing Windows at all costs". People who push this opinion are effectively no different to people who use FUDded comparisons to try and argue against Linux. If all you have to say is Windows is teh sucks, you come out looking like an idiot, and reinforce the opinion held by managers that Linux is not to be taken seriously.
Things that need to be linked from somewhere else, please: