I agree with Zerodark. That guy's there to troll, he probably has never so much as stuck a Knoppix disk in his CD drive. He may not have anything better to do than sit around all day picking arguments with people, but I bet you probably do. In that regard, let it go.
It might "hurt" to let someone badmouth Linux, but what are your options? First of all, it's not someone you know personally, so he's not going to influence the people you know best, and vice versa. Secondly, there is no "winning" argument to someone not interested in the facts. It's easy enough to find information on how Linux looks and works, and what it can do (much easier than XP, IMO), so if this guy isn't more informed it's because he doesn't want to be.
All your arguments will do is keep him going. A comedian (Mike Warnke) once had the perfect quote. "If you really want to get to a self-rightous idiot, just ignore the sucker." (Paraphrased) By ignoring him (or maybe plainly informing him that he should check his facts), you take away his thrill of trying to get under your skin. By arguing, you just feed that desire.
Better you should spend your energy doing something positive that supports your favorite OS. Answering questions on LQ is one, and positive advocacy (the nice, understanding, non-forceful kind) with friends, family, and coworkers will do more to change this guys opinion in the long run.
The ironic thing is, in a little while when Linux is THE OS of choice even among the average user, this guy's probably going to be bragging about how great it always has been.