2 years ago I bought an Alienware X51 R2 with the OEM Windows 7, 6 months ago I upgraded my pc to windows 10. A few days ago I upgraded my hardware and got a new motherboard, case,  PSU and Networkcard and now windows is asking me to activate windows with a product key. I know theres a product key on the back of my pc but its not valid, it only works for windows 7.  Where do I get my valid activation key?

I also posted this on the microsoft forum.