1) voldemort wouldnt entrust a piece of his soul to a creature that can think on its own, so nagini is out
2)The scar resembles the ring in that there was a lightning bolt broken out of the horcrux ring....when something is a horcrux it bears a lightning bolt on it somewhere
3)Voldemort killed harrys mother who was protecting harry.....the anger flowing through him must have definately been enough to make a horcrux with her death but probably didnt realize he had done it, then he must have touched harry in someway transferng the horcrux to him.(to make a horcrux you must focus hatred and kill while holding the object youre putting it in) harry gets the scar "A dark curse" this is the horcux
4)harry has all those unusual ties to voldemort, i.e. parcelmouth...
5)harry can see through voldemorts eyes, voldemort can see through his
6)and why do you think he wasnt in book 6 like at all, he had figured it out that someone was destroying them and didnt want to kill harry, again didnt the death eters have clear orders not to attack harry in book 6?
7)Voldemort doesnt want to kill harry he jus wants him kept away and alive via drought of living death, as discussed in previous books as the worst potion ever
8)the prophecy...harry must kill the piece of voldemort within him to kill voldemort