This article describes perfectly how to create a shared folder between a Virtualbox on an Ubuntu host with a Windows guest.

The article shows how to do it with Vista, but the same way of working is valid for an XP guest.

Just make sure to check the sharing options you have set on your Ubuntu machine for the folder you are trying to share! Make sure that sharing is turned on and that everyone has access to the folder.

How To Share Files In VirtualBox With Vista Guest And Ubuntu Host – Make Tech Easier.