I don't think the following is a recommended approach but I don't see any compelling reason for why that is so feel free to read on.
According to this answer it seems that you can make programs that are runnable on Windows Phone 8 (WP8) although it won't have all the features but probably most, and maybe the developer experience will not be the same. Also it does not seem to be an officially supported way ( I am not sure about that point ).
So it seems like you could use SDK 7.1 for example and update it to SDK 7.8. According to the System Requirements it runs on Windows 7. Keep in mind that I don't know if it is officially supported.