Hello hlamcknight,
Thanks for using Apple Support Communities.
If your computer does not fit the requirements for HD as the article below shows, then the SD version will be downloaded. To download the HD version, you will need to download it from your previous purchases.
When you purchase an HD video on a supported device or computer, only the HD video will be downloaded. To download the SD version, you need to download the video again from your Purchased page. Conversely, if you purchase an HD video on an unsupported device, the SD version will be downloaded. Then, you will need to download the HD version from your Purchased page. Learn more about downloading previous purchases.
iTunes: Purchasing and viewing HD videos
http://support.apple.com/kb/HT3209
Take care,
Alex H.