I had this same problem. Apparently I had time machine partitioning my drives without really knowing. I am a total noob when it comes to macs as this is my first mac book. Anyway, the file system is VERY different than windows and I had no idea that I really only had 25GB of free space left and was only really using 60 GB.. the rest was all time machine back ups. This was what was preventing me from downloading xcode. I saw a few of the comments here and downloaded daisy chain. You will have to pay for the licensed version, to do what you need to. So, I tried daisy chain and it is so easy to use and not only is my mac much faster but I am also after 3 days of trying to download xcode. Just make sure that when you go into daisy chain you click on the purple areas and that will lead you to the time machine back ups that you need to remove.