[KappaKittyCat]

Yeah, I was wondering about that, too. I mean, Voldemort says that he and Harry are both half-bloods. Now, Voldemort's mother was a witch and his father was a Muggle. However, Lily and James are a witch and wizard, so technically, Harry is not a half-blood. He says so in the first book when Draco asks about his parents.

Maybe Voldemort & Co. don't count someone as a wizard unless he can trace his ancestry back all the way through generations of wizards. That point alone makes me think that Lily was the first witch in the Evans family-- if Petunia were a Squib, or even if their parents were Squibs and Lily somehow turned out magical, then Voldemort would consider Harry a full-blood.