Windows should be on a primary partition else it will moan.
I don't understand what you mean with "invisible"? Are you sure you haven't deleted it? Can you see the partition from any of your other OSs? You can try dropping to the command line and type: fdisk -l /dev/sda (presuming your harddrive is sda) This will list all your different partitions, and you can find out where Windows is currently installed.
Also, maybe you can read-up about the command line for grub? Grub can read NTFS partitions, and you can play with what is needed in your config file by executing the instructions directly from Grub to get your windows running. This
looks like a good article, you need the "chainloader" command to get started.