I'm making the assumption here that anyone who reads this will know what Windows is: cheap, ubiquitous, and bad. First, Windows fulfills only the most basic OS requirements. Second, it is designed to be totally incompatible with Mac and Linux. And third, it's big: a Windows OS uses far more RAM than Mac or Linux. What this means is that I had Ubuntu Breezy Badger (Linux) running with 64MB RAM faster than Windows XP was with 128MB.

At this point only a few diehards really think Windows is the best system, but the use it because it's cheap, ubiquitous, and compatible with virtually all software.