She was filled with grace. That's the same as saying she was made sinless. The Orthodox hold that this happened at the Annunciation. The Catholics hold that it happened at her conception by her parents. Either way, she was made sinless the same way as we are made sinless through God's grace.
Our disobedience is made into obedience. Or as +Augustine puts it "[A]ll the commandments are considered fulfilled when whatever is not fulfilled is forgiven." [The Fathers of the Church, Vol. 60, Retractions, p.81]. We believe that once she was filled with grace she chose not to sin.
We are all made sinless by grace. That does not mean she didn't sin, it merely means that God has declared her righteous by her faith. The same as he declared Abraham righteous by faith.
Tell me kosta, are filled with grace? Are your sins forgiven? Are you righteous in God's eyes?
I have a memory from the L&E thread that is just barely strong enough to mention, but I thought that the Orthodox view was that Mary was filled with grace at around age 3, when she entered the Temple. I also want to say Palamas, but I have no quote. :) If I'm all wet then I apologize. I'm just trying to keep the differences straight in my head. :)