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).