If a normal uninstall can't be done, the only registry settings you might normally need to remove would be the Explorer Replacement ones:
Those would not normally exist unless the install completed, though.
In terms of files, it'd be the contents of C:\Program Files\GPSoftware\Directory Opus.
Settings also exist on disk, but probably not if the install failed. See How to backup or locate your Opus configuration (Simple) for a list of folders with config data.
If you need help getting the silent install working, please let us know more detail about how/where it is failing.