Question: I associated "Unknown" files with `notepad.exe`, that is to say files with no or unknown extension.

```
HKEY_CLASSES_ROOT\Unknown

```

However I am not certain of the keys I changed to that effect. I would like to restore all Keys and Values under `Unknown` to their "default" state, if possible.

Answer: 
I'm not sure what you're after, but this is a screen shot of my Unknown folder in my registry

![enter image description here](https://i.stack.imgur.com/coHps.png)
Deleting the following keys should un-associate whatever default `open` application you've selected for unknown type files:

```
HKEY_CURRENT_USER\SOFTWARE\Classes\Unknown\shell\open
HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Unknown\shell\open

```

They may not both exist, but that's ok. Leave the other shell verbs (probably at least `openas` and `opendlg` alone).