Windows NT, which has been the basis for modern versions of Windows (all versions of Windows Server starting in 1993, all versions of Windows NT Workstation and Windows NT, and all desktop versions of Windows since Windows XP released in 2001) is implemented with a very small amount of assembly language, a very large amount of C, and a smaller amount of C++ (primarily used in the GUI portion of the operating system).

If you’re focused on the kernel, it is virtually all in C.

The code is highly portable. It was running on non-x86 architectures through the 1990s, and in fact was first booted on a non-x86 architecture. Portability of the code remains a priority.

Some user mode utilities and applications that ship with the product are implemented in C#.