Could be a corrupt DVD. I have a 2007 too and it had 3P in it. I just had a friend download 7P from a Torrent and he burned it on DVD, works like a charm. All you need is a Dual Layer DVD burner and a DL DVD which is relativly cheap from NewEgg.
Hope this helps if you dont want to spend the money. I am selling my 3P for $40 too as no longer need it.
Thanks.