God's immunization of Mary from Original Sin does not mean that she did not have free will to choose whether or not to sin. She did. Instead, she chose to always say yes to God.

Like all of the Church's doctrines about Mary, the Immaculate Conception points to Jesus. He truly was God's son, thus Mary was conceived in a sinless state to prepare for his coming.

Source:
Youth Update Glossary
St. Anthony Messenger Press, 2002