The process is different depending on which hypervisor (virtualbox or vmware) you are using, but here is both solutions.
VMWare - http://www.vmware.com/products/converter/
Just download the converter and run it on the Windows installation you want to copy, and you can import that into any VMWare product, as well as Virtual Box
VirtualBox - https://www.virtualbox.org/wiki/Migrate_Windows
Download virtualbox onto the Windows installation you want to copy, then follow the directions on that page.
Either one of those should work.