Have you tried the Registry fix that may just allow the activation to go through? You can see a recent similar thread here:
i´ve just read and i found the problem is the number of times i've re-installed my FSX. I've done it two times and this is the third attemp. What I do want to take for granted is that i uninstalled it from the old pcs. It doesn't make sense to have installed FSX on an outdated pc. What can i do? My copy is legal and i just want to reinstall it on ONE pc, the new one.