Windows 8.1 is released only on retail copies or as a update on Windows Marketplace, you can not download it as an iso from anywhere except only if you buy it from an online retail store. Although if you still want a spare DVD to keep as backup you can download preview iso(links are below) and since they are iso you can burn them as a backup on a standard DVD. Its a preview by just name in reality its the whole Windows 8.1 iso, so don’t think you are downloading some half-baked iso. Mind you that even if the Preview iso is legal and free to download, product keys are not, you have to buy genuine product keys if you don’t have a product key Like i said, use this as a backup of your original Windows 8.1 DVD.
Product key: NTTX3-RV7VB-T7X7F-WQYYY-9Y92F (Preview Key)
Note: Aetomatic.com does not support piracy in any way, the preview copies of “Windows 8.1” given here are totally legal and free to download and after the installation they will give you an option to authorize it by buying or putting your existing Windows 8.1 Key, Product key given above is for preview versions only and is perfectly legal.