I did all that and it finished fine but then the BackTrack guest wouldn't boot any more and I got a message saying "Ubuntu is running in low-graphics mode" and also "(EE) Unable to locate/open config file". It then gave me a few options, one of which was to run the guest in low-graphics mode just this time (which is what I chose). I was then prompted to login. After logging in I typed the command sudo dpkg-reconfigure xserver-xorg and then chose Ok on all the options in the menu that followed. Once that finished I started the GUI again by typing startx. Everything is now back to normal.
Hope this helps anyone else that runs into this problem.
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