Assuming you really tried all the previous suggestion, there's a new one I've just read on AVSIM which, BTW, clearly confirm (together with your report that the problem started with a Windows Update) that is something related to Windows.
- Go in the Control Panel, under User Account, and change your account from Administrator (if it's of the Administrator type ) to "Standard user"
- Don't launch FSX yet, but reboot Windows
- Go in the Control Panel again, then change your Account type to Administrator again
- Reboot Windows again.
- Try launching FSX now.