Studio dashboard is showing Windows as installed, however there are no options to build for Windows until the path to the SDK is set in the preferences.
If I open Preferences->Studio->Platforms->Windows the Windows SDK is not set, setting this allows me to build.
1. On a Windows machine with Visual Studio and the Windows Phone 8.1 SDK installed open Studio
The Windows SDK is shown as installed but there are no options to build
The Windows SDK should not show as installed