Something must have changed then I guess. The plugin?
PF3 grabs the flight sim (Windows) handle (FS98MAIN) which is relevant for FS2004, FSX and P3D. It needs this for various things like using DirectX when playing sounds and setting FS as the foreground app after you hit 'connect', etc, etc. So it's quite important and PF3 cannot work without it. It may well be that with any FS where FSUIPC is involved it (FSUIPC) may change that name to the common FS98MAIN, but I really don't know. What the XP plugin does though I have no idea. Sorry