Much like any other windows application. All operating system vendors
publish the API to their operating system, otherwise no-one could
develop anything for it.

If you need the source for the kernel to write an application, then
there is something seriously wrong with the operating system's layering
model. The purpose of an operating system is to provide services to
applications, without the applications requiring internal knowledge of
the OS's resource management systems.

Of course they do, in the "Windows SDK" or whatever they call it
now-a-days. The NT 3.51 kernel and its derivatives (all modern windows
OSes) is Posix compliant, so the kernel API (instead of the  windows API
which covers all aspects of the OS) is governed by the same standard as
 most commercial unixes.

There are rumoured 'secret apis' which it is claimed Microsoft uses
internally to give its applications an 'edge' over competitors, but the
only evidence I'm aware of the use of non-standard apis in MS products
is that Outlook 2000 blue-screens NT if your disk partitions are greater
than 12GB, which doesn't seem much of an advantage over mail/diary
programs that use the published apis for file system access.

Pete