There are many possibilities.
You can use something that can capture your desktop such as VLC, FFmpeg, Java, or the camera utility that came with your computer.
A more bandwidth efficient solution would be to store the powerpoint in some representation on the client and then control switching of slides remotely. You can export Powerpoint slides to jpeg, HTML, SWF. You can control remote clients with RTMP RPC (through Wowza), or with your webserver using polling, Ajax or HTML5 Websockets.
You could probably even use .NET and the Powerpoint application natively.