Not sure if this is your problem, but it could well be.
FSX can only be used on one machine, it checks its self with Microsoft.
A new OS may be giving the wrong info. In which case you might need to contact Microsoft by phone.
You may find more info at the following forums:-
I used to use FS9 but found my machine would not cope with FSX, so gave up on it. When I buy another PC then I might try again.
FSXs programming was too far ahead of hardware capabilities at the time.