Cubase SX3 problems

We have an old computer running Windows XP, especially so that we can still use Cubase SX3, however, recently this computer has had a few problems, so we have bought another running Windows XP also. We though it would be an easy thing to do, just put the old hard drive in a usb case and run it from that, however, this apparently requires an E-licence, which we do not have, we’re assuming this is because it is running through the USB port and not installed directly. We have also lost our original disc, so are unable to install it onto the new computer. The new computer housing is not big enough to run the two hard drives side by side and we are now at a loss what to do. Have any of you, any suggestions how we can get the system up and running. Thank you