To over a billion people, Microsoft has an OS that "makes sense". There is a reason Microsoft is king in the enterprise market, and it isnt solely because of the cheaper prices. It is also because of the quality and choice of software, the Microsoft Ecosystem is great, especially in the enterprise market, where everything needs to be compatible with each other. Windows today is very secure, most of the big virus swarms are caused and spread by the kind of people who will click accept to any dialogue box, open any attachment they receive.
Imagine if a business was run entirely on Leopard, how screwed are they now? Apple seems to have left Leopard in the dust, or what if they'd just upgraded to Snow Leopard because they could afford it, and most of their business was dependent on software that required Rosetta, what will they do when Apple inevitably stops supporting Snow Leopard soon. Custom designed Computer Software for Enterprise is incredibly expensive to have coded, and many businesses can ill afford to have it upgraded and remade.
At least with Microsoft you can have the support you need, Microsoft are still supporting XP, and companies can still make an agreement with Microsoft to continue receiving support till beyond XP's shelf life.