From VirtualBox manual, paravirtualized network adapter provides the best performance. To get this interface from working within VirtualBox, you need to download the drivers separately/manually. Note that Windows driver online update utility does not have it as well.
Since driver is open source it can be found in few locations. A good source is from Red Hat, download it from this location http://alt.fedoraproject.org/pub/alt/virtio-win/latest/images/. Download virtio-win*.iso, mount it and install driver for your OS.