by removing that, it is fixed! thanks for youre help! enjoy youre day!
AH! Got it, it's NOT the registry, it's your dummy FSX.EXE with a 0 length!
That's very dangerous, because if you really wanted to create a dummy .exe to fool installers not made for P3D to go into there, it should have been a real FSX.EXE or, at least, a real .EXE named FSX.EXE, because LOTS of installers (including all our own), don't just check if the file is there: they MUST open it to check its *version* because they must know if it's FSX, FSX SP1, FSX SP2, FSX:SE so, with a lenght of 0, it's surely an illegal .EXE, which will make lots of installers to fail.
But I'll try to add this case too to the already long list of problems in the users installations our products must defends against from.