Well, if you are downloading from a mirror it's not out of the question that you got a bad download both times! More importantly, you're not following the advice in answer #1 or you would at the very least know if you had a bad download or not before burning the disc. This link also covers the other things that are likely going wrong such as burning at high speed, and will tell you a way to get a good download faster tha from the mirrors and how to be 100% certain if your download is good or not.