I have an iPhone, iPod and iPad and I have downloaded the SD versions of HD movies from itunes to all devices. I just tried to download some of the same movies to our old laptop and it won't allow me to. My guess is that if your computer can't play HD it won't allow you access the movie at all. In my opinion it's Apples way to discourage use of non apple devices/computers. However, I still love my apple products,
If you go on the ipod in "itunes", then, "purchased" then "movies" you will see the list of downloads available.
You can search on itunes for the purchased movie and the click on download or if you want the SD version, just scroll to the bottom and click on the link that says "Also available in standard definition": This will significantly decrease the amount of space the movie takes on the ipod. Whnen yo scroll back up you will see that the download has switched to SD.