Recently I had to work with a Microsoft Project Plan (.mpp) that was sent across by one of our partner vendors for communicating their plan. I hunted a bit for the best way to open and review this mpp file on my Ubuntu 8.04. Then I found OpenProj.
It can import and show mpp files pretty easily and can export them in M$ project 2003 XML format. I have not tried reading the exporting file on a M$ windoze system, but I guess it should be fine.