You shouldn't have to do anything special on Windows 7. The Windows 8 Kindle app is a different beast altogether. The books aren't stored as a single file with the app. If having access to the ebook files is important to you then uninstall the app and install the regular Kindle for PC program on Windows 8.
If you don't see the books in your My Kindle Content folder on Windows 7 (and the program is registered to your Amazon account, and you've clicked on books in your library to download them), I can only assume that the library folder has been changed in the options. Check the options to see what folder the program set to use.
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