Quelle version de VirtualBox ? OSE ou Oracle ?
As-tu installé les Additions Client sur Windows ?
As-tu regardé la section Shared Folder de la doc
http://download.virtualbox.org/virtualbox/3.1.6/UserManual.pdf
4.7 Folder sharing
Shared folders allow you to access files of your host system from within the guest
system, much like ordinary shares on Windows networks would – except that shared
folders do not need require networking. Shared folders must physically reside on
the host and are then shared with the guest; sharing is accomplished using a special
service on the host and a file system driver for the guest, both of which are provided
by VirtualBox.
In order to use this feature, the VirtualBox Guest Additions have to be installed.
Note however that Shared Folders are only supported with Windows (2000 or newer),
Linux and Solaris guests.