The natural-law argument states that because there are consistent and predictable natural laws in the universe, there must be a law-giver who set those laws in motion. That law-giver is assumed to be God.
This argument relies on equivocation between two meanings of the word "law".
Legislative laws, such as "Do not murder" or "No littering" are prescriptive: they are established to demarcate acceptable and unacceptable behavior. If a person breaks such a law, he or she has committed a crime, and may be subject to punishment.
Natural laws, on the other hand, are descriptive: they describe how some aspect of the universe behaves. For instance, Newton's law of motion "F=ma" describes how solid objects behave when acted upon by a force. If a person or object breaks a physical law, then it is the law that is in error, since it obviously does not adequately describe what it seeks to describe.
Natural laws as human concepts
Bertrand Russell wrote:
"We now find that a great many things we thought were Natural Laws are really human conventions. You know that even in the remotest depth of stellar space there are still three feet to a yard. That is, no doubt, a very remarkable fact, but you would hardly call it a law of nature."
Where did God get the laws from?
Similar to the question of "Who created God?", there is the question of whether God was in some way required to choose the laws that he did.
"Why did God issue just those natural laws and no others? If you say that he did it simply from his own good pleasure, and without any reason, you then find that there is something which is not subject to law, and so your train of natural law is interrupted. If you say, as more orthodox theologians do, that in all the laws which God issues he had a reason for giving those laws rather than others -- the reason, of course, being to create the best universe, although you would never think it to look at it -- if there was a reason for the laws which God gave, then God himself was subject to law, and therefore you do not get any advantage by introducing God as an intermediary."
- — Why I Am Not a Christian