Having a complete Windows (or Mac) desktop running within Linux has been possible for some time now, thanks to the wonders of Virtual Machine (VM) technology. However, the typical approach is to mount and boot a VM image, where the guest OS and hard disk are just files on the host filesystem. In this case, the guest OS can't be natively booted and run, because it doesn't occupy its own disk or partition on the physical hardware, and therefore it can't be picked up by the BIOS / boot manager.
I've been installing Windows and Linux on the same machine, in a dual-boot setup, for many years now. In this case, I boot natively into either one or the other of the installed OSes. However, I haven't run one "real" OS (i.e. an OS that's installed on a physical disk or partition) inside the other via a VM. At least, not until now.
At my new job this year, I discovered that it's possible to do such a thing, using a feature of VirtualBox called "Raw Disk Access". With surprisingly few hiccups, I got this running with Linux Mint 17.3 as the host, and with Windows 8.1 as the guest. Each OS is installed on a separate physical hard disk. I run Windows inside the VM most of the time, but I can still boot natively into the very same install of Windows at any time, if necessary.