Assassins Creed I – Why?

Hi guys.
Just brought a new PC, and I decided to try my assassins creed I Directers cut (borrowed from a friend) legit version.
Yet I started the game, and it runs in a window!
Is this normal?
If not normal… how to fix?
Answer #1
Go to video options, choose between Windowed, Borderless Windowed, and Full Screen. These are normally the 3 options available.
Answer #2
Is this in-game? or on the menu?
Answer #3
click ingame and press alt+enter
Answer #4
When the game window is open, press alt + enter