Make sure you install the software as an administrator in the Win 8 environment and also it is best to have FSX in it's own directory (i.e. C:\FSX) to avoid permissions and folder issues within Windows 8.

To install the software as an administrator, please do the following:

1. Go to Start Menu
2. Go to Accessories
3. Go to Windows Explorer
4. Go to My Computer
5. On the left highlight your CD/DVD drive
6. On the right you should see a setup.exe
7. Right click on that and install it as an administrator
8. After installation is complete, reboot your PC, then start up the application

If you are still unsuccessful, then make sure your User Account Control (UAC) is turned off in your system settings.

http://blogs.msdn.com/b/hyperyash/archive/2012/07/18/disabling-user-account-control-in-windows-8.aspx