If my understanding of Islam is correct, Muslims believe that the Bible [or at least the Tawrat (books of Moses), Zabur (Psalms of David) and Injil (Gospels of Jesus)] was originally given by God. (Correct me if I'm wrong.) They believe that the Bible was afterwards corrupted (words changed in order to significantly alter the meaning) so that it is now unreliable as a source of divine truth. The Quran, they believe, is the final revelation from God, and the only one that has not been altered or corrupted in any way.
My question is, do Muslims believe the Bible was corrupted before the Quran was written down, or after the Quran was written down? If any of my Muslim friends who read this would be so kind as to leave a comment explaining the traditional Islamic answer to this question, that would be great.