When installing Prepar3D fresh:
1/ In the Prepar3D settings you may want to export the controls to a saved file.
2/ In Windows Control Panel, choose Uninstall a Program, goes to Programs and Features. Locate the Prepar3D Client and choose uninstall but choose to preserve the license. Then choose Uninstall Prepar3D program (not contents) so that all the items are gone from the window. You may have to restart the Programs and Features window to check all items are in fact gone.
3/ Rename all the folders:
C:\Users\you\AppData\Local\Lockheed Martin\Prepar3D v4
C:\Users\you\AppData\Roaming\Lockheed Martin\Prepar3D v4
C:\Users\you\Documents\Prepar3D v4 Add-ons
C:\Users\you\Documents\Prepar3D v4 Files
C:\Program Files\Lockheed Martin\Prepar3D v4
C:\ProgramData\Lockheed Martin\Prepar3D v4
I rename them all by adding " old" to the end of the path e.g. "Prepar3D v4 old"
By renaming the folders you have a backup of the \Prepar3D v4 Add-ons folder and also any other items you may need. Prepar3D will create new folders and install fresh.
4/ Run Prepar3D and set up anything you need like importing the controls and making settings.
5/ If you are using HT enabled, set up an Affinity Mask. Example added to C:\Users\you\AppData\Roaming\Lockheed Martin\Prepar3D v4\Prepar3D.cfg:
[JOBSCHEDULER]
AffinityMask=1365
6/ Add the Modify Allow permission to the Prepar3D folder:
In Explorer choose the Prepar3D program folder "C:\Program Files\Lockheed Martin", in the menu choose Properties, choose the Security tab, press Edit, select the Users group (MyComputer\Users), check the Modify Allow box, OK, OK.
Now you are ready to install addons.