Christians believe that some of the Old Testament is the word of God, but not all of it. They believe that it was corrupted in the 2000 odd years between it and the New Testament. The reason that it is included in the Bible is because it gives us our history, the book of Genesis, Noah, Job, Moses, etc. However, only the words of Jesus are the true words of God, so Christians only believe what Jesus confirmed to be true, like the ten commandments.