Fallacy of the Excluded Middle:
All cats are animals
All dogs are animals
Therefore, All Cats are Dogs
Addressing an argument with a personal attack constitutes an example of the ad hominem abusive fallacy.
ad hominem abusive is a class of ad hominem fallacy.
Calling into question the person making the argument, and not the argument itself constitutes an example of the ad hominem circumstantial fallacy.
ad hominem circumstantial is a class of ad hominem fallacy.
Therefore (by the fallacy of excluded middle): All ad hominem circumstantial arguments are ad hominem abusive ones.
And so, all ad hominem circumstantial arguments are personal attacks.
Now, concerning what constitutes an ad hominem circumstantial:
ad hominem as a class is a fallacy that involves arguing against the person (ad hominem) making the argument and not against the argument itself (ad argumentum).
Lazs' example of the blind man seeing isn't one, since in that case, the blind man is part of the argument: "I, person X, saw Y" is invalid if Person X is incapable of seeing.
On the other hand, if someone is arguing, "Invading Iraq is a bad idea because A) there's no evidence of WMD and B) It will only engender generations of hostility towards Americans", the fact that that person dodged the draft, has frequent sexual liaisons with farm animals, and wears Axe body spray has nothing to do witht the argument.
ad hominem circumstantial is the most common form of ad hominem on bulletin boards, ("I don't care what michael moore says; the very fact that he said it makes it untrue")
Other forms of ad hominem are the aforementioned abusive, "A: We should not invade Iraq. B: A is an ******* and a terrorist", and the always popular tu quoque ("you too"), as in, "A: We should not invade Iraq., B: but A invaded Vietnam". The question isn't what the other person did or is doing, but rather what should b edone.