Considered by many Protestants to be the hardest of the Marian dogmas to accept, the Immaculate Conception is not something clearly spelled out in Scripture. Of course, just because something is not explicity stated in the Bible does not mean it is not to be believed. However, the Immaculate Conception of Mary, that she was in the first moment of her conception preserved free from the stain of original sin, is actually centered in the first recording of the Gospel, the proto-evangellium.
In Genesis 3:15, the motif known as the "Seed of the Woman" is born. "I will put enmity between thee and the woman, between thy seed and her seed. He will bruise thy head, and you will bruise his heel." In God's conversation with the serpent we rightly emphasize the latter half of this prophecy; God's promise that at the Crucifixion of the Child of the Woman the head of Satan would be definitively crushed. Yet it cannot be overlooked that the Lord also states that enmity will exist between the serpent and the Woman. This is not in any way a clear statement that Mary was immaculately conceived, but if Mary was at emnity that means she was a foe, an opposite. What is the opposite of utter sinfulness? You guessed it.
In the end, I still think the best argument for this dogma is that Christianity has always taught that Christ takes his entire human nature from his mother. If he is perfect, and he is, how can his mother not be perfect? How would Christ not be infected with her sin?
Sancta Maria, ora pro nobis