I know that Draco Malfoy's parents are both wizards so he's full blood, but maybe there's more to it than that or maybe there's some secret concerning his birth. J.K. Rowling hinted in that interview I read online, that the half-blood prince, whoever he is, is one of the contenders in the chamber of secrets. So, since it's not Tom Riddle a.k.a. Voldemort and not Harry either, there was one person there whom they thought of to be the heir of slytherin, and it was draco malfoy, right? I don't know, maybe this idea is just too farfetched, but... well... it just, sort of, made sense to me.