Have you run into a problem when the installer says on Configure Installation that AutoCAD is already installed and trying to clear out remaining folders, files and registry keys does not help? AutoCAD is not installed but still the setup says it is. It might be that you had tried to manually uninstall AutoCAD (or Revit or whatever it is) and failed to do it 100%. What to do?
This trick might help to solve the problem. Note that the GUID (the code at the end in italics) is different for different applications and versions so you might need to search in the parent folder or go through them all to find what you look for.
Locate this folder and delete it with its content. Make a backup if you want to be on the safe side.
Use regedit, locate this registry key and delete it with its content. Make a backup if you want to be on the safe side.
After that hopefully you will find that you can run the setup and install AutoCAD or whatever product you had in mind to install. Now you know how to install AutoCAD even if AutoCAD is already installed even though it was just the installer thinking so.