i am using virtualbox under linux windows 7 doesn't support the drivers for the provided network interface for virtualbox
PCnet-FAST III Am79C973
the driver can be downloaded via: http://www.amd.com/us-en/assets/content_type/utilities/V4.51.zip
create an iso-image and mount it with virtualbox
mkisofs -r -o driver.iso /<driver/directory>
install the driver for win2003 !