Ubuntu is probably installed in the 14.75 GB OEM Partition shown in your image since the others are all NTFS (which means Windows) or much too small.
I think you are trying to delete the 117Mb System partition (which is probably diagnostics software for your PC).
While you may be able to delete the Ubuntu partition, doing so may render your PC unbootable. If your PC boots via
grub in the Ubuntu partition (which is likely), deleting this partition will stop the PC from booting up at all as
grub won't find it's boot files, so won't be able to boot either Ubuntu or Windows!
If you are confident about it, then instructions for replacing
grub's bootloader by the Windows bootloader can be found here. http://www.linuxbsdos.com/2012/03/10/restore-the-windows-bootloader-to-mbr-after-dual-booting-with-linux/
However, since it's only using 14.75Gb of your approx 400Gb disk, I'd recommend just forgetting about it until such time as you are doing a reinstall of Windows (when you can just reformat the whole disk).