Don't be fooled into thinking that because your user account is an administrative account, that you are running FSX and/or P3D as an administrator. With Vista, Win7 & Win8/8.1, you are only really running them as an administrative if

1) you right click on the FSX/P3D shortcut and select the Run as administrator option:

http://i669.photobucket.com/albums/vv58/Markoz958/runfsxasadministrator_zpsb234943f.jpg~original

2) right click on the FSX/P3D .exe files, select Properties, select the Compatibility tab, then checkmark the (place a tick in) box beside Run this program as an administrator (under Privilege Level):

http://i669.photobucket.com/albums/vv58/Markoz958/runfsxasadministrator.jpg~original

It is highly recommended NOT to install FSX (and I assume the same applies to P3D) in the C:\Program Files or C:\Program Files (x86) because Windows has too many restrictions on these folders (and everything within them). You are better off installing them in the C: root, i.e. C:\Microsoft Flight Simulator X and C:\Prepar3D v2.

My FSX and P3D are install on the F: drive in Windows 8.1 (because Win8.1 is installed on my F: drive): F:\Flight Simulator X and F:\Prepar3D v2, and they both work fine.