This command really saved me a lot of time. Its easy enough sharing folders when both the host and guest machine are windows (Network Neighbourhood is used)
But, when the host is windows XP and the guest is ubuntu you need the command:
sudo mount.vboxsf [share_name] [directory – mount point eg /mnt/share/]
So in my case for example I had to use (as root):
mount.vboxsf SharedwithVM /home/david/SharedHost
where SharedwithVM is the name of the shared folder I defined in Virtual Box
via Virtual Box Shared Folder between WinXP host and Ubuntu guest : Taos Sun Blog.