HP Final book, lesson #1: the future Is Not Set On Stone.
Harry Potter prepared to deliver the killing blow, was hit on the side of the head knocking him out. Draco who was to blaime quickly spun moved his wand approprietly uttering the Killing curse before Voldemort could process what had happend. In one fell swoop Draco had killed the mortal enemy of both Wizzards and Muggles, as well as stole all the fame and glory that Harry Potter would have gotten. Harry who was in the end very greatful to Draco for having taken him out of the lime light, became very good friends with that man. Harry Potter could now live a normal life as the man who almost killed Voldemort, Voldy for short.
As for Draco, after his friendship with Harry Potter blossomed he got to know Hermione, Who after realising that being in a relationship with Ron would never be mentaly stimulating, and thus was now single, began to make his advances to win her over. Harry had discovered that Draco had taken a fancy to Hermione since third year when she had punched him (a Machosit perhaps?). Harry now realised that that had also been around the time that Draco had stopped bullying Hermione, decided to help his new friend and his old friend Hook up by playing match maker.
Two year later they welcome their first child a little girl named Rosen and not a year later a little boy named Scorpious.