[Solved] Just installed purchased FSP, FSX won't start

[whiskey-zulu]

You need to remove everything - you won't have to mess around in the registry - you just need to make sure that all the files have been removed
(including any freeware add-ons you installed ourself manually) are removed.

Yes, installing to a different root file path will cause problems - you should in all circumstances install to C:/program files/microsoft
games/fsx as FSX has problems reading for anything that isn't in that root path.