Huh. Running as admin is the only thing I know if that interferes with u functionality, unless maybe it's an AV program or a file permission thing. I'm on Windows 10, though, so I can't say definitively that there's nothing up with Windows 7.
Can you maybe give us a screenshot of where your dragging from and what happens when you drag to the library?
Are you saying that scanning for new files doesn't work, either? "Add files to library" I think is only meant to do single files, whereas scan for new files should get whole directories.