This is the definition of ad hominem. For those like me who didn't know.

An ad hominem argument, also known as argumentum ad hominem (Latin: "argument to the man", "argument against the man") consists of replying to an argument or factual claim by attacking or appealing to a characteristic or belief of the person making the argument or claim, rather than by addressing the substance of the argument or producing evidence against the claim.

More on wikipedia http://en.wikipedia.org/wiki/Ad_hominem