As above, you have two options:
1) Install Windows as a VM inside Ubuntu, provided your system has enough resources. This is advantageous because it allows you to run both OSs at the same time and access all software on both systems without having to reboot. The downside is the computer needs to have enough resources to run both OSs at once, and Windows may have issues with direct hardware access if you need that (rare).
2) Repartition your hard drive to open up some space, install Windows in that empty space, then boot a live Linux system and reinstall GRUB to repair the boot sector.