It has become very fashionable among those who would attack the Church to hurl accusations of paganism at every turn. In almost every instance what these accusations betray is a fundamental lack of understanding of the transformative power of the incarnation. God became matter, sanctifying all matter. We cannot be at war with God's creation without being at war with God himself! A tree cannot be "pagan" - it is part of the divine creation of God.
First, I would point out that the claim of Christmas trees being of pagan origin is dubious. There are old writings referencing the use of evergreens at Nativity in pre-schism times. Moreover, simply because an item had a pagan use in history does not equate to the conclusion that the same items used in the Church are pagan. Pagans had altars where they offered sacrifices to their God. Are we to remove the altar from our Church? God forbid! Pagans used water and oil in their rituals. Are we not to baptize and anoint? The misuse of matter sanctified by Christ taints it only in that use. It does not make non-pagan usage pagan. To be sure there may be some things that we would not use or do so as not to scandalize, but a Christmas tree is not one of them, given its near universal recognition as a Christian testament to the celebration of the Nativity of Christ.
What we are certainly not to do is worship Christmas trees. Nor are we to give in to the consumerist worldly temptations denoted by piles of presents under the tree. That is another type of worship, and one to be avoided!! If you cannot separate the tree from the commercialization of the Nativity, then by all means, do not put one up. But don't do it because of the thought that there is some pagan stigma attached to it. I'm aware of no Orthodox councils anathematizing Christmas trees (although in full disclosure, I haven't researched it). If the Church has not seen fit to lay such a ban, why do you believe as a layperson that such a ban would be proper? We are neither commanded nor banned to put up Christmas trees. Do as you wish, but don't make your decision because you believe a pagan taint applies. That is bad Christology.