The obvious advantage:
Windows Server is developed and sold by Microsoft. The company continually provides support and updates. Users who are familiar with Windows will find themselves comfortably using Windows Server.
Most people using Windows Server hosting utilize Active Server Pages (ASP) technology. This is the main reason why people are turning to Windows Server hosting.
Users can develop a web site using familiar Microsoft tool interfaces such as Microsoft FrontPage, Visual Interdev, and Microsoft Access.
With ASP, users can develop a database-driven web site using Microsoft Access and Microsoft SQL as the database.
The disadvantages are:
Unlike UNIX, Windows Server requires more system resources. You need a powerful machine to run Windows Server.
Windows Server does not have a good reputation in terms of server stability. Windows Server needs rebooting more frequent than UNIX.
If you are using Windows Server hosting services, you may find yourself investing a lot of money in development tools and most of them are Microsoft products. The costs of the applications that can run on your web site are usually higher than UNIX.
For example, you can find a lot of free scripts to run web board, chat room, web stats, email (and more) for your UNIX-based web site, but you won't find many free applications in Windows Server world.