go to itunes store on pc, go to the purchased section of the itunes store and click the movies tab. Look at the bottom right of the window and make sure the "Download HD When Available" box is not checked and then try downloading the movie and it should only download the SD version.
This option is no longer there on the latest version of itunes! How in the world do I download the sd version of movies now once I purchasr the hd version.??? Hello apple, I hope you didnt take this feature away as the hd versions take too much space on my ipad mini.