1. Install Guest Additions in VirtualBox.
  2. Create folder on Windows host.
  3. In VirtualBox, add a Shared Folder. Give it a name and choose the folder you created on the host.
  4. Open the VM. Create a directory; this will be the shared folder location on the guest.
  5. Run the following command to mount the host shared folder.
# 4. Create a directory on the guest
$ sudo mkdir ~/virtual
# 5. mount the host folder
$ sudo mount -t vboxsf folder-name ~/virtual # where folder name is set in VirtualBox shared folders dialogue, ~/virtual is the folder created on the guest in the last step.

You could use the /etc/init.d/rc.local script to execute these commands on startup to have the shared folders automatically mounted every time you start your Ubuntu VirtualBox.

More info here