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On the Sourceforge project page I can see that win32gui is covered by the Common Public License. Moreover, in every (at least as far as I know) source file there is an information about the copyright (however nothing about the CPL). I think it can be a bit confusing, especially because I couldn't find any separate license text included with the source distribution. Maybe it would be a good idea to clarify things by adding such a file and also putting some information about terms of usage on the project's main page?
I have added a file license.txt. Hopefully that will help a bit.