I was installing Ubuntu 13.10 and chose to replace the old Ubuntu 12.04. The problem is that the new installation also erased my Windows and installed the new Ubuntu using all space available in my HD, instead of keeping the dual boot.
That also happened with my friend, we are both developers that need both OS. Very sad, this should be fixed.
EDIT:
As @oldfred said: "Reinstall says overwrite Ubuntu but it also erases existing Windows. bugs.launchpad.net/ubuntu/+source/ubiquity/+bug/1265192"