You have to mount the windows partition first.
I would probably do it all on the command line, working as root so you get all the access you need. I'm guessing what the windows partition is (sda2) and where the virtual disk is (wubi/root.disk). You will have to adjust accordingly:
So now your linux filesystem is visible in /mnt/wubi. It's probably easiest to remain root to copy the stuff off as you don't then have to worry about the ownership of the files you're reading. You can just run the command nautilus from the command line where you are root, and nautilus will start, with full root access.
mount -t ntfs /dev/sda2 /mnt/windows
mount -o loop /mnt/windows/wubi/root.disk /mnt/wubi