Switching off Virus software did the trick. Strange that with Virus control on, GSX installed into FSX no problem?
Maybe you had your antivirus exclusions set-up in the FSX folder, but not in the P3D folder.
What I usually do, is to exclude the ENTIRE sim folder from antivirus scanning, to get a faster loading time, otherwise the antivirus will try to scan each and every executable ( .dll, .exe and .gau files ) at every startup of the sim.