Solved: Using Wise Program Uninstaller's "Force Uninstall" feature (the normal one didn't do anything, since all files had already been removed) found two registry keys and removed them. Problem solved.
Okay, I could have searched the registry myself for the string, but I wasn't sure if that was all, and since I had previously destroyed a Windows 8.1 installation by removing registry keys I was reluctant to do so.