The DLL itself is included with the OS. Microsoft does not provide an actual driver themselves for any particular hardware, but all official Windows OpenGL drivers from all graphics hardware vendors like AMD/Nvidia/Intel are required to link against the Microsoft OpenGL libraries and call functions from the OpenGL32 DLL (at least in the initialization stage), in order to maintain a consistent level of standardization/quality. They are of course also allowed to implement their own extensions on top of it, however, which are generally contained in additional vendor-specific DLLs.
Also, not that this is particularly important, but just in case you didn't know: ATI was acquired by AMD in 2006, and AMD stopped using the ATI brand name in 2010.
EDIT: I should further clarify that there is not even really such a thing as an "OpenGL-specific driver" in the context of Windows. OpenGL support is simply one part of any given driver from any given hardware vendor (the other part being DirectX support, of course.) You cannot "install OpenGL" or "uninstall OpenGL" on Windows.