You can't.  Youtube is a website, and the videos have been embedded in the page for playback using Flash.  Media Player cannot play Flash files, and is designed primarily for downloaded/downloadable media.  If you download a MOV, MPG, WMV, etc., then it will play in Media Player.
I have Windows vista and internet explorer 9. I have set windows media player as the default program for playing videos and music files. However, when i play files from Youtube the files are played in Adobe Flash Player. Why is that? and how can i change it so the files in Youtube are played in Windows media player?
set the default media player in windows vista 