The Fanboy defends a particular product under any circumstances, no matter what the scenario. He is fervently aware that the Ford Pinto was an excellent car as long you avoided rear-end collisions in it. He also assumes that anyone who fails to praise said product to the skies must be engaged in an irrational vendetta against it--in effect, he sees everyone who isn't a Fanboy as an Anti-Fanboy.
The most common Fanboy tendency is to worship either something (or everything) Apple-related or something (or everything) Microsoft-related. One unmistakable sign that you're dealing with a Fanboy of this kind--and not with just an enthusiast--is that, even though the object of his veneration is ultimately a product for sale, he somehow manages to subordinate technological and monetary considerations to moral imperatives.