Windows 7 product keys are found on a sticker on the computer case, in the product packaging or in the Windows registry. The product key in the registry is encrypted, but free key locator programs can find and decrypt the product key.
In cases where none of these methods finds the Windows 7 product key, Microsoft provides replacement product keys for legitimate installations. This replacement key is not free, but is less expensive than purchasing a new copy of Windows 7. The computer's manufacturer also has access to replacement product keys for computers that were purchased with Windows 7 pre-installed.