The windows 7 is a windows 7 key, not a Windows 10 key. It was only the first year that you could upgrade a win 7/8/8.1 to 10 free of charge AFAIK.
So, the best thing you can do, I think, is to just reinstall windows 7. On the installer, when it ask you to choose a partition, you have a button to format it. Then put your existing product key when the installation program ask for it and you should be good to go for windows. After that you'll just have to reinstall the drivers for the hardware