You can't slow the videos down while they're on YouTube but you can on your own PC:
Save the video to your PC using Keepvid:
http://keepvid.com/
(Paste the YouTube address into the field surrounded by a green box. Click 'Download'. Click 'Download link' and choose a location for the file. Preferably name it with an 'flv' file extension, e.g. 'dance.flv').
Now you need a player to view it. Download VLC (for free) from here:
http://www.videolan.org/
Launch VLC and go to File > Quick Open File. Navigate to the location, on your hard drive, of the video and double-click on the file name to play the video. Click the 'double left-handed arrow' button (the right-hand one of the pair) to slow the video. Click it again to slow it further.
Chris