did you somehow overwrite your Ubuntu partition with your windows install? Where was you ubuntu installed to previously? What you can do is use the live cd and view the folders that contain your ubuntu installation, most likely in /media/ and if they aren't there, you can mount them yourself to any folder that you create anywhere and see if your ubuntu installation is still there. if not, then I am sorry to say but you overwrote it within windows. are you maybe chosing the wrong disk? do you have more than 1 disk?