Talking Java to PowerPoint is one problem. Google for "java com bridge" for a variety of solutions. JACOB is a good open-source choice. The POI library has custom connections to several Office products, don't know if it does PowerPoint.
Driving it from the server is quite another. Will you have a real Java client or a browser? A custom Java server or a standard servlet container? Some combinations are easy, some would be better done with ActiveX than Java.
A good question is never answered. It is not a bolt to be tightened into place but a seed to be planted and to bear more seed toward the hope of greening the landscape of the idea. John Ciardi