By default, Power Point lets you change slides by pressing keys on the keyboard, or by clicking a mouse. The simplest solution would be to use an arduino leonardo to emulate a keyboard or mouse.
Actually the simplest solution is to crack open an old keyboard and rewire the keyboard chip accordingly. This is trivial. It is also very simple to add some transistors to make the Arduino "push the buttons". Of course it is not considered very elegant but it is definitely much simpler than USB programming.