Arduino Yun, Leonardo, Esplora and Due have the ability to appear as a native Mouse and/or Keyboard to a connected computer by use of the Mouse and Keyboard libraries.
Here's an example of how you would use SWITCH3 on the Esplora to active PAGE UP on your keyboard.
if (!Esplora.readButton(SWITCH_3)) {
Keyboard.press(KEY_PAGE_UP)
delay(100);
Keyboard.releaseAll();
}
Here's an example of using the Esplora to control the Keyboard and Mouse to control the Flight Simulator option of Google Earth.
If you want to use the Uno it's more difficult but it can be done by sending data from the Uno to a Python program running on the PC. The Python program uses the pywin library to control windows.
Here's a fully worked out Instructable on how to do it.