A windows7 serial key is one of the two things you need to make your windows genuine; the other one is windows loader.
Try to search for those in search engines; do this at your own risk.
HIGHLY RECOMMEND: JUST BUY A GENUINE WINDOWS7 OS.
RISK OF DOING-IT-YOURSELF WITH LITTLE KNOW-HOW ON HOW IT WORKS; Your whole system would just crash and you will need to format your PC.