I had Windows and installed Ubuntu alongside. Somehow GRUB wouldn't recognize Windows, so I erased the Windows partition and attempted to reinstall it, however I got error messages like "the partition is not windows-compatible". My partitions:
/dev/sda1 - Ubuntu/dev/sda5 - Ubuntu swap/dev/sda6 - A previously installed Ubuntu version-not using it anymore/dev/sda7 - only data, no OSI could potentially delete all partitions if needed except my data partition (sda7).
Use gparted to resize the Ubuntu partition, and then create a new partition with the NTFS file system. Then install windows from the CD to that partition (it will automatically pick that up as the partition to install to). You may need create an Ubuntu liveCD, and use it to reinstall grub, as windows will have replaced grub with its bootloader. (I can provide a link on how to do this if necessary). TL;DR make sure there is an NTFS partition somewhere on the disk.
