Hang on: I thought Voldemort is a half blood because his Mum was a witch and his Dad wasn't (Seamus calling himself half and half in the 1st book), BUT Harry came from two magical parents and he is a half blood!!!!!! Therefore the definition of 'half blood' has to be based on a longer genealogy than just your parents. I've lent my copy of the 2nd out but I think there is a passage about Harry justifying his blood because of his parents' magical status but I can't remember how that particular conversation ends.

V could only have chosen Harry as the vanquishing threat because of his half-bloodedness not Harry's status as orphan because Harry wasn't an orphan when V chose him- follow my circular logic? Therefore Harry wasn't an orphan with any kind of sharp impliment to grind or do anything else with.

Additionally, D says that according to V's "creed", purebloods are "the only kind of wizard worth being or knowing" but he chose a "halfblood like himself. He saw himself in you [Harry,] before he had ever seen you" (p.742 Aus. hardback, OotP). Thus, V has an inferiority complex about his blood and a superiority complex about his blood all at the same time.......my head hurts

Dumbledore tells us that Neville is a pureblood and Harry's a half blood (p742, Aus. hardback, OotP).

OK the mysterious spider salesman:
Why, Zonko, do you think that this TRAVELLER "obviously" moved into the Hogworts area?
Here's a thought: Tom Riddle semed to know a lot about Hagrid's goings on, enough to know exactly where to find Hagrid to get him expelled. How about the spider peddler was Tom in disguise stirring up some trouble (a la Quirille)? I am making all this up and don't actually believe a word of it but, sorry Zonko who I love now as a brother, I'm finding EVEN my theory more believable than your's right about now. Give me something else to go on, a prickle of a smidgen of a hint to back you and I'll be there for ya