While this re-tread argument gets tiresome when you see it year after year, many Christians really have no idea how to respond to this (although simply holding to the Biblical record serves as a safeguard in all matters of faith). As a result, and as a desire to not be thought of as brain-dead, brain-washed fools, Christians are tempted to throw away the faith.
Lutheran Satire to the rescue! Here is a brief, entertaining explanation of all those pagan myths supposedly worked into the Christmas story. Enjoy!