I have only just picked up on this thread.

How to access a shared folder in an Ubuntu Vbox guest, hosted on a Vista
system:

1/ Create the folder you want to share on the Vista host

2/ In the Vbox GUI, go to the settings for your VM and move to the
Shared Folders section. Add the folder you want to share and remember
the name you give it.

3/ Start your VM and, when you're logged in, create a mount point for
the shared folder

4/ Open a terminal and enter the following command:

sudo mount -t vboxsf name-you-gave-shared-folder /path/to/mount/point

This works in Ubuntu 9.04. The only problem is that, unless you keep
the folder open in Ubuntu, the mount does not seem to be persistent.
I'm sure someone can tell you how to add this to fstab to get around
that problem but, personally, it's not important enough for me to be
that bothered about it :-)