You have two harddrives right? It is possible to do that, but you have to do some work on the console. With Virtualbox you first have to create a file describing the Harddrive you want to partition:
VBoxManage internalcommands createrawvmdk -filename file.vmdk -rawdisk \\.\PhysicalDrive1
file.vmdk will be the resulting descriptor file and PhysicalDrive1 is your first HD on Windows. If youre unfamiliar with the command line look that up. You probably have to change the directory to where VBoxManage is located. And you have to use an administrator shell.
From there its easy. In Virtualbox create a new machine, assign the Gparted iso as a virtual CDrom, the file.vmdk as the harddrive, boot from CD and there you go.
But extra careful what you do! If you manage to partition your system drive, you'll probably loose a lot of data. But its possible, I installed Linux on a second partition (not second HD) that way.
Good starting points for further research are
virtualbox raw disk access windows PhysicalDrive