hi big dan
the serial number should be on the plastic holder the cd came in. without that number I think you are out of luck on it. you might ask gp if you send them the disk if they could somehow correct the problem and allow you to use it. don't see why they would not if you have the original disk. which version is it?
another possibility, if you still have your old computer I think that number is stored in a file either under real flight directory or in your c: root drive.

good luck and happy flying

pt19