It appears the non-PRO version was not fully removed from your system and that is why the installer is preventing it from continuing. My suggestion is to install the Steam version or to manually remove the entries from your registry to install the non-Steam PRO version. To get the Steam version, use this page: http://www.rustangelo.com/steam_claim.php
To clean the registry and install the non-Steam PRO version, open regedit and expand the HKEY_LOCAL_MACHINE node. Navigate to: Software/Microsoft/Windows/CurrentVersion/Uninstall. Do a search for Rustangelo and delete the node that contains it. You may also have to delete any entries located in: Software/Wow6432Node/Microsoft/Windows/CurrentVersion/Uninstall.