MinWin is the layer of core components for the Windows operating system (OS). It runs the kernel, file systems, TCP/IP, hardware abstraction layer and other core services. MinWin has evolved from projects within Microsoft over the years to reduce interdependencies within Windows.
Techopedia explains MinWin
Microsoft began using MinWin as the basis for OS builds in 2006-2007, when Microsoft replaced Windows XP with the Windows Vista. An implementation of MinWin was part of Windows 7, and it is used extensively in Windows 8. The basic objective behind MinWin is to minimize the number of running services to just those that enabled the operating system to be self-sustaining, and perform its role as an OS.
MinWin is all about reducing the core of Windows to a set of basic functionality. The basic MinWin in Windows 7 is comprised of about 161 files, whose total footprint on disk is about 28 MB. To accomplish its task, MinWin makes extensive use of virtualization within the OS. Application level calls are made to virtual DLLs that in turn are mapped to logical DLLs. This allows a separation and modularization of the core API sets of MinWin.