harry is not a pureblood cause he's mom wasn't. it got explained in the end of cos i believe. but you're forgetting about the weasleys who are all purebloods and in gryffindor and james potter and sirius black were in gryffindor as well.
i think the reason why tom riddle was put in slytherin is because he's slytherin's heir. i think i've read others say that before in some other threads and i agree with them.
i guess as a pureblood you can be in any house depending on what your strenghts are. and even tho the weasleys are pureblood i don't think think they are "worthy" to be a slytherin in salazar's standards.
the kind of blood you have does not define which house you belong in. You get put into a house because you show the trates that the house is known for. . but muggle-borns (we don't know about half-bloods i don't think yet) are not sorted in to slytherin because (i'll use the same word impat used. . ) they are not worthy to be in sytherin by other sytherins eyes