I already have Ubuntu installed on the machine and a spare NTFS partition.
If you just install Windows onto your new machine it will overwrite your bootloader, so you won't be able to boot into Ubuntu anymore. Generally it's better to first install Windows and then Ubuntu, as Ubuntu recognizes your Windows Installation and creates a Grub-Entry to start it.
However, this way around works too.
You will need some sort of Live-CD (Like the Ubuntu Install-CD). Boot it.
Mount your Ubuntu partition: (REPLACE X and Y)
sudo mount /dev/sdXY /mnt
sudo grub-install --root-directory=/mnt /dev/sdX
Now restart your computer. Grub should now load normally, without an entry for Windows. As soon as you are back into your system type
to add Windows to your bootmenu.