This is the third time I've done this now and I keep having to look up the detail here so I am going to write it down this time. In my case I need to refresh the install of Windows 8 Enterprise trial every 3 months when it expires as I don't yet have a full win8 copy. You'll find the trial ISO on Microsoft's site and the next think to do is to prep a bootable USB stick with the OS. Note that the link to the Windows 7 USB/DVD download tool in Hanselman's article is broken as I write this and I don't need it so if starting from scratch you'll need to locate this or an alternative. I'm not so I don't!
The important bit for me is to F10 from the disk selection step of the install to pick up my existing VHD and fire up DISKPART from the cmd prompt. Once in:
DISKPART> select vdisk file=d:\VMs\Win8.vhd
DiskPart successfully selected the virtual disk file.
DISKPART> attach vdisk
If you do not already have the disk created you'll need to do so first:
DISKPART> create vdisk file=d:\VMs\Win8.vhd type=expandable maximum=60000
or similar. See the earlier section of the referenced article.
In my case I also need to reformat the drive before proceeding and skip over the errant 'can't install to this disk' warning.
Then let the Windows install do it's stuff and you should then get the dual boot option on startup.