Either when it first starts (called a cold boot), or when you tell it to restart (called a warm boot).
I always find I have to repeatedly press F8, rather than keeping my finger on it, because I have never been sure exactly when to do it. It is before the Windows logo appears.
, when the screen is black with white writing. It is F8 on every computer. When you get to the boot option screen, there will be a list of ways to start, pick safe mode, not safe mode with networking.