I've been using FSX on Windows 8 since Jan 2013. Had a few problems with FSX crashing and also errors in finding the UIAutomationCore.dll file. I followed these instruction on removing and reinstalling FSX. And it been running great since.
Although I kept my saved flights folder by renaming it and then copying it back after the install was complete.
Also this link here on SIMforms has a lot for good info as well, but I think the main thing is to make sure FSX is installed in a folder on your c: drive such as c:\fsx,
I also had prblems reinstalling FSX getting a error 1722 just beforre the install completed. Microsoft has a software reset tool at this link:
And it worked ok in Widows 8 on my PC.
I tried upgrading to windows 8.1 and ran into problems with very slow frame rate. And then I found I couldn't delete FSX in Windows 8.1. Kept geting an error during the uninstall. Fortunitly I did a complete back up and restored everything back to Windows 8. I might try windows 8.1 again but will remove FSX first before I do the upgade to Windows 8.1 (some time when I have 2 or 3 days to waste messing about with it.