It is a feature of NTFS (the filesystem) so really you create it once (using a command in the command line or a utility to run it for you and you are done) and the program sees the data as being in it's original location but it occupies no space on that drive. If you need more info if you pm me the folder location and the path where you want to move the files to I should be able to pm you the cmd to create the junction or you can use this app:
http://www.rekenwonder.com/linkmagic.htm
I can't personally vouch for it, I just found it in a quick google. You would need to completely close itunes first, even the crap that runs in the background. Then copy the folder to its new home and delete the contents of the origional. Right click on the app and run as admin then click create. select the original folder on the left and the new folder on the right. Should do the trick.