Good tutorial.
This step:
As root, put this line in /etc/rc.local
mount -t vboxsf -o rw,uid=500,gid=500 linux /home/yourusername/Windows
Would be achieved better by adding the following to /etc/fstab:
# mount windows share
linux /home/yourusername/Windows vboxsf rw,uid=500,gid=500 0 0
rc.local is a bodge when nothing else will work or for testing purposes.