Use function keys on a Mac keyboard in Windows with Boot Camp
By default in Windows, pressing function keys on a portable Mac and some other Apple keyboards controls hardware features. Boot Camp supports the volume, brightness, keyboard illumination, and media keys.
To control app features, you press a function key and the Fn key. For example, if you open iTunes and press Fn-F1, iTunes Help opens.
In the Boot Camp Control Panel, you can change the default behavior, so that pressing a function key controls app features, and pressing a function key and the Fn key controls hardware features.
In Windows, click
in the right side of the taskbar, click the Boot Camp icon 
, then choose Boot Camp Control Panel.If a User Account Control dialog appears, click Yes.
Click Keyboard.
Select or deselect “Use all F1, F2 etc. keys as standard function keys.”
Some portable Mac computers have a group of keys with small numbers on them that can be used as a numeric keypad. To use these keys as a keypad, press the Num Lock (F6) key, or hold down the Fn key while you press the keys on the keyboard.