I know in previous versions there have been issues with users who have installed and uninstalled Autodesk products on their machines in the past. Sometimes noise is left in the registry that kills AutoPLANT installations. You can check the registry keys located at:
HKEY_LOCAL_MACHINE\SOFTWARE\Autodesk\AutoCAD
See if there are any entries for products you no longer have installed. There is an article with a bit more detail here.