I have an Ubuntu Bootable Installation CD.
How to install from the CD to a VirtualBox on Windows after creating a virtual machine as described here.
The link mentions about downloading Ubuntu disk image (iso file), but I would like to skip this step as I have a bootable CD already.
Can I suggest, put your bootable Ubuntu disk into your CD/DVD drive.
At the point where you attach the ISO image as per the instructions, choose the drop-down to select the CD/DVD drive on your Windows box.
Start the Virtual Guest, it will then boot using the disk in your CD/DVD drive.