This guide details my experience trying to run my dualbooted Windows install as a VirtualBox guest under a Linux host. This procedure was tested with Windows 8.1 Pro and VirtualBox 5.1.2.

1. Download vmdk-raw-parts¹ and follow the instructions. You'll end up executing something along the lines of²:

   
   ./vmdk-raw-parts /dev/sda ~/WindowsVM 1 2 3 4
   
   

   if your Windows partition is /dev/sda3, and various recovery partitions are 1, 2, and 4. You now have a directory with a .vmdk file and some other important files that direct VirtualBox to your physical Windows partition.
2. Start up VirtualBox, set up a new VM and point it to the .vmdk you just created. Important settings are:
   • System → Enable IO/APIC
   • System → Enable EFI if your physical install boots from UEFI
   • Network → Adapter 1 → Advanced → Attached to: NAT, Adapter Type: Paravirtualized, Cable Connected: true
   • Display → Screen → Enable 3D Acceleration: false (this may work for you, but it made the virtual screen a weird size for me and created a massive cursor offset)
3. Boot your VM and cross your fingers. If you run into any problems I recommend checking out amhill's guide as linked below in the References section.
4. If you boot up successfully but have no network access, download the stable virtio-win ISO from Fedora³. Pop the ISO in the virtual CD drive, open Device Manager, find the Ethernet Controller under Other Devices, right click Update Drivers, browse locally, and point towards the CD (the checkbox "including subdirectories" should be true). If all goes well, Windows will prompt whether you want to install the "Red Hat VirtIO Ethernet Adapter." Accept and you're on your way!

References

• amhill's guide

Windows