Creating a powerpoint file format viewer in Unity is probably not terribly feasible. I'm sure it's possible (after all, Microsoft did publish a file format specification), but I doubt you'll find an out-of-the-box managed plugin or anything like that.

As for video, the only thing Unity supports at run time is ogv files. You can stream a movie texture using the WWW class. Depending on your needs, you might have to write a headless transcoder for arbitrary files people upload.

Unity can play videos by default if you have Pro. You should convert videos to ogg theora format on your server or tell clients to do this.

For playing a powerpoint slideshow, you should read the standard schemas of powerpoint file format and try to simulate it using unity GUI, which by no means is an easy work. It needs hours of coding. The file structure is not that hard, but implementing all effects in Unity is not that easy.

I might not understand your question well, but I think instead of Unity you should use other packages like Silverlight.

If I want to talk more about videos, you should create a Unity editor script and use it with a Unity headless server or any other converter program to convert videos - then you can download them by Unity's web player using www class and its movie property.

Silverlight is, however a better package to do what you want, I think. In there, either you can display Powerpoint files or you can convert slides to html pages and show them inside the Silverlight app window.

Elearning packages can do what you want out of the box. Just google the term "free learning management software".