In Windows all the new APIs since Windows Vista are mostly COM (C++) based, including the new Userspace Driver Framework.
With Windows 8 this will increase, as Microsoft is moving to have C++ as their official systems language, by introducing WinRT as the new subsystem, and dropping support for anything besides C89.
Just because C is used a lot, you cannot say that all (even 99,9%) operating systems use it.
Actually I failed to mention the amount of operating systems that you may have running on your watch, vcr, radio, microwave, etc, which summed together are much more that the OS X, Windows, Linux base.
OS X and Windows have some C++ (mostly wrappers) but otherwise are C.