Not totally sure still as I asked 2 questions and you answered yes
You are just trying to make a disk image of your old drive to use in virtualbox?
If that is true then either the program I suggested or the one redk9258 suggested will work. The one I suggested will create a virtual box virtual HD (VDI). The one redk9258 suggested will create a virtual pc virtual HD (VHD). Both types can be read from virtualbox. VHD can also be mounted to look like a real drive in windows 7.
If you use the one I suggested you will need that drive number (as per the docs of the program). Use the command prompt and run the diskpart the give it the list disk command. It PROBABLY is #1 and #0 is your current C drive. But that is not guaranteed, so make sure you look.
The one redk9258 is little more point and click. It will image the whole drive as a VHD and create it on your C drive wherever you tell it.
If you are trying to create a backup using windows backup. You need to use the 'let me chose' option when you are setting up the backup. That will let you pick which drive to image.