If you have Ubuntu installed along with Windows, you will have Grub as the bootloader. So if you decided to remove Ubuntu, then restoring the default Windows bootloader is required (else you can keep grub, but that's not the point), which you must do at first.

The easiest and the least time consuming way to restore Windows bootloader is to install lilo bootloader under Ubuntu. And do a restore as shown below.

sudo apt-get install lilo
sudo lilo -M /dev/sda mbr

Now you have updated the MBR (Master Boot Record), and it will boot into Windows. You can use any hard disk management tool to format the GNU/Linux partition back to NTFS.