The Public folder is a shared folder by all users on the computer. It is highly recommended to not move it. Doing so, will break all shares, and may not work in Windows Media Center any more.
Instead of moving your user profiles and Public folder, and possible messing your installation up, you might consider to create new folders on the external drive, then include those new folders in libraries
of your choice. You could even set a new folder to be the default save to location of a library
. This way everything is saved in the the new folders on the external drive, but your user folders and Public folders are still on C without the risk of messing up their junction piont links.