This is often controlled by hardware directly, Not Windows, there will be buttons on the screen display settings. These change between Screen models. Older CRTs will have Dials instead of buttons. Most Laptops will have some Button combination to alter your screen settings, and may have alter brightness based on the power settings. The Button Combinations will usually be FN plus a sun symbol(☀).
Depending on your hardware and the installed Drivers, you can alter the brightness and a few other settings within "Graphic Card" settings, which will usually mention AMD, ATi, Nvidia or Intel.