Activation of Windows 10

My pc came with pre installed windows 7 ultimate. Then i upgraded to windows 10 anniversary update, however I didn't liked it and rolled back to windows 7. Then i installed windows 8 pro from a recovery cd that came with my pc. After installation i faced problems in activating it since my pc manufacturer didn't gave me a product key. Now i have windows 10 pro 64 bit and i don't know how to activate it legally. Can you help me in getting back activation details for my pc so that I could get all features of Windows 10