I am currently using Ubuntu 14.04 and I want to install Windows. But I don't want my Windows files to be accessible from Ubuntu and vice-versa. I want to work as if I am on two different systems though using the same hard disk.
Is it possible to create a stand-alone hard-drive partition?
How inaccessible do you want them to be? Linux has FAT and NTFS drivers and there are ext2/3/4 drivers for Windows. Anybody with administrative privileges or physical access will always be able to gain unrestricted access to the data of each file system unless it's encrypted.
If you're afraid that the two operating systems will somehow intermingle: they don't¹. In a sane set up each system will live on its own disk partition(s).
¹ unless you make them with a significant amount of effort on your side.
