[mesquite]

I'm in the process of getting a new computer but I'm not sure of the differences between Kindle for PC on the different operating systems of Windows 7 and 8.1

In my notion is that Kindle for PC on 8.1 is an app where the books are stored in the "cloud" thus making them inaccessible to add to calibre. Is this correct?

[Eros]

I have both operating systems. Kindle for PC runs and looks identical on both systems (whether you open Kindle for PC through the app or from the Desktop on Windows 8.1)
In Windows 8.1, you can still load Kindle books (from My Kindle Content) to Calibre the same way you do on Windows 7.