In order to use the Guide button in Windows and to prevent Steam Big Picture from starting up, you have to take some additional steps.
In Windows: go to Start > Settings > Gaming > Game Bar and turn off everything there.
Download the file below, this will filter out Guide button presses when you have steam running.
This file is just a simple DLL file which you have to place in your steam folder
Default location: 'C:\Program Files (x86)\Steam'