There are 2 ways Windows sets what is being displayed on the Projector.
The first way Windows might do this is by Extending the Displays. This means that the Projector's screen and the Computer Monitor Screen are completely separate. Meaning you can show a Power Point Slide Show on the Projector, and on the computer monitor you can have your notes.
The second way is by Duplicating the Displays. This means that the Projector's screen and the Computer Monitor Screen are exactly the same.
Here is how to change these settings:
1.) Go to the search bar on the start menu and type "Display Settings".
2.) It should bring up the following Menu. Scroll down till you see Multiple Displays.
3.) Click on the Multiple Displays Drop Down and Select either Duplicate or Extend (whichever you prefer).