To enable windowed mode you have to change two lines at the beginning of your f1graphics.cfg file :
0 ; f1graphics.cfg controls all settings (default 0)
800 ; [RELEASE] Game 3D Horizontal Resolution
600 ; [RELEASE] Game 3D Vertical Resolution
1 ; [RELEASE] Enable 32bit mode if available NB. If Windowed then is forced to desktop format
1 ; [RELEASE] Enable hardware T&L if available
0 ; [RELEASE] Run in a window
...
Set the first line to 1 to enable settings control
Set the 6th line to 1 as well to run in a window
But before that you should try to set the second and third line to your screen resolution and have a try with it. Don't forget to set first line to 1 for that.