Well, first, you will need to be able to download the video off of youtube. Search firefox add-ons, as I'm not sure IE has such an add-on, but I could be wrong. There was one called youtube video converter or something, just look around.
Then once you save the file, you click the "Insert" tab at the top of powerpoint, and on the far right you will see "video". Click it and select "video from F
ile...". Browse to the location of your saved video, and then select it, and click "Insert". There it is. Size it, etc.
Don't use any copywrited material, as that can put you in a world of hurt.
*Edit - sorry Influx, you posted before I was finished typing. Beat me to it