A valid Retail edition product key.You can download a Windows 7 ISO from Microsoft here and make a bootable USB key with a utility called Rufus. You will need a valid product key.
OEM ones will see it refer you to your hardware vendor for a disc.
And comically the XP one was moaned about by people who prefered the cleaner look of the 9x one, which was moaned about by people who preferred the simpler interface of 3.x xDThe Windows 7 Start menu was long regarded a mess too by people who wanted the XP style one.