In addition to what the others have said, there is a very big list of 3rd party vendors who use C#. Look at enterprise CMS, for example. Microsoft also has a very large footprint in both CRM and ERP systems. Those install bases and the ISVs who build on top of them use Windows. There's also the micro/small business market where a server resides in the office to handle printing/video surveillance, small ERP apps. Think about dentist offices/clinics, shops, restaurants, gyms, small warehouses.