What is the programming language of Windows 7?

The foundation of Windows 7 is mostly written in C and a little bit of Assembly.

This is because a relatively low level language is required for access to processor registry management and connection with other devices (e.g. printer) to be established to the computer.

Most of the other core stuff is written in C++.

Lastly, there is a bit of C# written on the .NET Framework here and there, but the use of it is not that abundant (yet).