There is Camera Mouse, which lets you use your heat movements to control the mouse pointer. Not exactly what you asked about, but still in the general vicinity. By recognizing head movements and turning them into mouse movements, this gives you the ability to activate many different programs, giving you camera control over many windows comands.
You could use Win&I gesture control software for WIndows 7 and Kinect. To specifically address your request regarding the possibility of having body movements recognized when using a powerpoint presentation, Win&I has software specifically for using a camera for controlling a powerpoint presentation. Click that link to be taken to the product page.
GestureTek is working on multiple different solutions, and even has an SDK for programmers.
This should be a more complete answer now, considering it specifically addresses exactly what you are asking for by providing a link to software that is made for exactly what you are asking for.