Many computers from major manufacturers (Dell, HP, Lenovo) have the key "injected" into the motherboard's BIOS. You can retrieve it using free tools like Belarc Advisor or ProduKey.
Before looking for a "free" key from a stranger, check if you already own one. Many older laptops and desktops came with Windows 7 pre-installed. free windows 7 home premium product key
To clarify why searching for free Windows 7 product keys is unsafe and illegal, and to outline legitimate alternatives. Many computers from major manufacturers (Dell, HP, Lenovo)