I think you mean embed the video?
YouTube encodes its videos as Flash Video, to download the video you can use a number of online youtube websites such as http://www.videodl.org/ or http://www.techcrunch.com/get-youtube-movie/
The format you will receive is a flv (flash video), you will then need a third party program such as ImToo Video Converter to convert the flv into a normal avi or mpg video, which you can then embed into any application, including Microsoft Word and Powerpoint.
This is done by using the object packager. http://www.indezine.com/notes/2004/08/using-object-packager-to-embed-video.html
Another method is demonstrated with this video...