Windows is a sophisticated, robust operating system that is comprised of several subsystems. Programmed using the Win32 API, it provides a mechanism that allows user applications to (indirectly) make calls into the system kernel code.
Windows provides an architecture that protects the system from being destabilized by errant user applications. It provides a virtual memory facility to alleviate the need for apps to implement their own virtual memory managers. And it provides memory-mapped files and copy-on-write support for making advanced memory management easy and efficient.