There's a documented "internal" function that you might be interested in. You can create a "raw disk" image which basically passes all commands to the partition on your hard drive. That way, you can keep your Windows partition and attempt to boot it from VirtualBox as well.
You can create a raw image as follows:
VBoxManage internalcommands createrawvmdk -filename /path/to/file.vmdk -rawdisk /dev/sda -partitions 1,5
where 1,5
means partitions 1 and 5 on /dev/sda
Mind you, the chances of this working flawlessly are pretty slim. Windows is not exactly known for being adaptive to extreme changes in hardware.
You can read more about raw disks here: Chapter 9. Advanced storage configuration10.8.1. Using a Raw Host Hard Disk From a Guest