What is really interesting about Archer is the reason, or well trying to understand why he changed that much from his youth. Shirou wasn't properly a brilliant guy in society but he had good manners and he was caring a lot about others. He was a kindhearted person willing to sacrifice himself for others, ready to help everybody who could need him.
Archer is very different under this point of view. He still has the will to help others (something that he's indeed trying to suppress) but he's desperately seeking for the hate of others. He doesn't want to have more regrets in life than the other he alreasy has. He deeply hates himself for his vain dreams, for having condamned himself to a fate of sufference, to a cycle he could never stop. He seeks peace (in death) and this is the miracle he's hoping for. Shirou want to find that peace in life instead...that's probably why I still consider their personalities quite similar, although they could appear very different. Their relationship is complexed as well. We know Archer wants to kill Shirou, it's his mission and the only way in which he could get his one true wish realized. He appearently doesn't need the help of the Holy Grail, but I consider his partecipation to the war as something that still implies his attachment to life. I think that he doesn't really want his annihilation and that he unconsciously hope to get his happinness in winning the war and getting the prize, but these are only my opinions and he doesn't probably realize his real intentions until the end of Unlimited Blade Works. Anyway Archer gives Shirou many hints since the beginning of Fate. I think he still believes he can change his destiny by acting in his past and he matures the idea to kill Shirou later on, when he realizes that his determination is too strong to be annihilated. He's charmed and suprised by the strenght he thought he lost in growing up, in facing a reality that is very different from the world of justice and righteousness he has always hoped for. He symbolizes the extreme regret of mistaken choice although he states that he can learn only from his own mistakes. He hasn't changed that much than, he doesn't really want to admit he's the same person back then. Although he lost all his will to carry on and his love for live he still has the hope to be happy.
Since archer is only one of the possible evolutions of Shirou, there are some points that should be underlined about these different developments: Archer wasn't able to save Saber (and this involves the fact that Avalon was probably removed from his body in an other way or maybe never removed) nor Rin or Ilya and he lost all of them. He hadn't probably a so physically strong relationship with none of them and probably he tried to become a member of the Magi Guild since he recalls some memories of an experience lived in England. He has never become a Magi though.
Due to the fact that at that time he wans't wishing for his self destruction, he wasn't probably featured as Rin's servant as well (so it means there was an other possible archer at his place).