When you open the Kindle for PC application you just installed, what does the title bar say in the upper left corner. It should say something like "David's Kindle for PC 2". When you go to Content and Devices => Devices =>Amazon apps installed on devices, you should be able to remove any Kindle for PC entries that do not match the name from the title bar. Under more actions, you can set that device as your default unless you are attempting to avoid KFX downloads with newer versions of the K4PC app.