In Vista, Windows 7 and now Windows 8, it is not recommended to install FSX into it's default location due to the severe restrictions put on C:\Program Files and C:\Program Files (x86).

You could try to override it by Right Clicking on FSX\fsx.exe > Properties > Compatibility > Checkmark (tick) Run this program as an administrator (under Privilege Level), so it always runs with administrative rights.

Personally, I think it's better to keep out of C:\Program Files and C:\Program Files (x86). ;)