How have Protestants explained how Jesus could be born of mankind and be sinless? Very simple question really. If the sin of Adam is passed down through the generations, so that all of us are born with a sin nature, how can it be that Jesus was born of Mary (a human being), yet sinless?

I'm from a Protestant (specifically Baptist) background, and would be interested in knowing how we explain it. Most churches have rather vague doctrinal statements that don't address subtle issues like this. So I am wondering if there are any common answers to this issue. Good answers will show that the position proposed is accepted by more than just the poster. 

The only argument I've beard before is that sin is passed via the seed of man, but I find no Biblical support for this, and it sounds like an artificially constructed argument to oppose some Catholic answers to this question which may require the sinlessness of Mary (which would be contrary to the Bible from the Protestant point of view).
The Bible is very clear that Jesus did not begin to exist in the womb of Mary.  As the Second Person of the Trinity, He exists outside of the physical universe of time, space, and matter and with the Father and Spirit created the physical world.

He is holy, was holy, and will always be holy... and sinless and righteous and pure and blameless.

When Mary conceived, the eternal Son of God became flesh and entered into the physical universe.  That act did not require that He become sinful.  He retained His holiness, as He is the same yesterday, today and forever.

His being born of a virgin is an important part of this, of course.  The doctrine indicates that without a human father, Jesus did not inherit the sin nature.

When He was born, He was given the name of Jesus (The Lord saves) because He Himself was the Lord who was saving His people from their sins.