Install Windows 7 without Product Key and Activate Later

I want to buy a legal or genuine copy of Windows 7. How can I get it if I have a gaming computer?
I have tried to install my friend’s copy of Windows 7 but I failed to install it since it doesn’t accept the product key.
Can I install Windows 7 even without inserting the product key?
And then, just activate it when I already purchase a genuine copy of it.
I will appreciate if you help me in this matter!
