My brother is giving me old parts, and I'm going to put my old hard drive in with them. He did the same with his new parts and it asked him to delete partitions before he could install Windows. After that he had to put in his product key. I think I'll have to do the same, which is why I'm wondering how to obtain my product key if I never got one from a disc. I bought the PC pre-built from Best Buy and it already had Windows 8.1 on it.