Microsoft made Windows 8.1 installation media reject Windows 8.0 product keys during installation but not during activation. The solution is to install with a generic product key and then obtain your product key using RW-Everything. You can then activate using change key. See here for more details: [URL]  
About a year ago I bought a dell inspiron 15r with windows 8 single language installed.Dell didn't send me a windows installation disk and I don't know my product key.I didn't like windows 8  back then and made a clean windows 7 installation and used it for a year.Recently I decided to reinstall windows 8.I found a windows 8 iso file and installed it via my usb stick.It supposed activate my windows 8 automatically but it didn't.During the installation it didn't ask my product key,I thought it was activated but it isn't.The question is can I find my windows 8 product key with my service tag. The iso file I found is windows 8.1 single language,I think there is no problems there.
Can't activate reinstalled windows 8