Although the question requires Windows 7 compatible software, there isn't anything special about the question that makes it Windows 7 specific. Thus, it seems like the windows tag is more appropriate.
My understanding is that if a question asks for recommendations that will only work on a specific OS version, we use the tag for that version. When the recommendations do not require functionality only in a specific OS version, but must be compatible with a certain version, we use the most generic tag for that OS, and mention which versions with which is must be compatible within the text.
See also: How specific should the OS tag be?