Hi Philip,
I did the same. Upgraded my desktop PC yesterday to windows 8.1 from windows 8 (fsx + pmdg 777 worked perfectly).
Now my fsx crashes. Using menus -> crash, using PMDG 777 -> crash, using full screen (5 screens for me) -> crash ...
If it can be helpful, we are not alone, most of new windows 8.1 users can not play FSX anymore. Some of them has uninstalled FSX then a full fresh install with same issues.
I ve updated my video card driver, same issues.
I ve tried to get back to windows 8. Impossible. i ve tried to use a windows 8 backup but windows 8.1 doesn't accept to downgrade from a windows 8 backup. If anyone knows how to do so, i will really appreciate.
Windows 8.1 seems to be a trap. I was about to upgrade my laptop too, i will wait.
I hope this is only a graphic driver issue if not, indeed, windows 8.1 will have kill FSX (I'm asking myself if Windows will refund FSX + add ons lol )