Split a partition with GParted

You cannot shrink a mounted partition. Boot from a live CD/USB and from there run gparted. This way you can shrink the partition. After the install, you must use this live media again to repair grub.

Edit:

1. Back up your files.
2. Create the live media if you don't own it already.
3. Boot from it, and run gparted.
4. Select the partition, and click resize, which will bring you to a new window.
5. Grab the bar at the right end with the mouse, and shrink the bar.
6. Now execute the task, it will take time, because first it will do some tests and later it will move some data.
   2. If a message pops up that you might ruin the boot process, you should not execute the task. (If this happens, then you need to repair grub.)
7. I suggest, that when it is over, you reboot.
8. The new partition can be created from the normal ubuntu.
9. After you installed windows, do come to this site, and you will find how to repair grub, which was overwritten by windows. Here is the solution for this: How can I repair grub? (How to get Ubuntu back after installing Windows?)