Although I have not watched the Fate/Zero TV series, I have watched the Fate/stay night series and the Fate/Stay Night - Unlimited Blade Works movie. At the end of the movie I found out that:
Archer is actually Shirō. Even before the ending, Archer told him he has already experienced what Shirō goes through. So I did not know if he is actually Shirō from the future or if he is a completely different character than the one in the TV series.