How do you get free windows -7 product key?

You can't ! Each unique copy of Windows 7 is pre-installed on computers. The licence key for the legitimate copy supplied with the computer is 'woven' into the Windows registry. If you try to install another copy with a different licence key, your computer will claim it is an illegal copy - and display various messages warnign you of that fact.

You can buy a legitimate copy from Microsoft - but it will (a) need to be installed from scratch, (b) register itself with the Windows server and (c) disable the licence key that came with your original copy.