You can use GCC and Clang directly on Windows, both implement 100% C99. Alternatively, Visual Studio 2013 implements most of the C99 standard (not a complete implementation though).
You don't need to install MinGW or MSYS in order to use GCC (I assume you are interested only in C99 here and not in POSIX programming). Here is an example of a standalone GCC (contains C, C++ and Fortran compilers):
