Native code in Windows is mostly written in both C and C++, in that you will see both .c and .cpp files, but since all DLL interfaces in Windows are C interfaces (C++ interfaces basically require you to be using the same compiler, though you can use COM to wrap C++ classes to build portable OO interfaces), even the .cpp files tend to be more C-like than C++-like.
There are C++ developers at Microsoft who do very ninja C++ things. But for the most part, people using .cpp files might as well be using C - many people just use .cpp so that they can declare variables in the middle of a scope. Some people have a class they internally use to do state management or implement some algorithm.
Basically, the common reason for use of .cpp is to get language features that increase readability. Very little in richly decorated in C++-but-not-COM faces outward, even in small internal components. I won't say none, because I never conducted an audit to check.
Source: me, worked in Windows for 4 years and change.