PyGTK cannot find my python 2.7 folder on Windows

I am trying to install Python 2.7 PyGTK 2.24.2 on Win 7 64bit PC. My python is 2.7.1 [MSC v.1500 64 bit (AMD64)].

I downloaded pygtk-all-in-one-2.24.2.win32-py2.7.msi from the PyGTK site, but when I run it, a pop-up appears: "Python 2.7 cannot be located on your system." And the installation failed.

Is it because my python is for x64? Or do I need to set up any environment variable (which I already did for PYTHONPATH)?

Thank.