Installing windows after ubuntu is installed (need to create ntfs partition too)

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http://askubuntu.com – I prefer Ubuntu to work on, but for some applications for school, I need windows. I've done some research and have a few problems: 1) I only have one hard drive which is formatted as ext4, not ntfs. Many people say I have to use Gparted to create an ntfs partition to install windows on, the only problem is that I need to unmount my drive to do this, which isn't possible I think because I only have one. I thought about mounting my HDD as a slave on a windows laptop but I don't manage to partition it with windows partition manager, so I think I need some kind of tool. which one? 2) next proble (HowTos)