After updating virtualbox to version 3.1 today, I faced the problem of having to update the guest additions that come with the new version on a headless xp guest. Here’s the command line to attach the guest iso to the xp guest instance:

VBoxManage modifyvm rfactor --dvd /usr/share/virtualbox/VBoxGuestAdditions.iso

Make sure to use the correct path to the vboxguest iso file (this example is from CentOS 5.x).

Hope you find it useful, thanks to Sasquatch from the virtualbox forums for the help!