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: 
You can pull default registry settings from
[Windows 7 ISO](http://mydigitallife.info/official-windows-7-sp1-iso-from-digital-river)
without even reinstalling Windows. The following will create a Virtual Hard Disk
to save the data you get from the ISO

```
diskpart
create vdisk file C:\wo.vhd maximum 10
attach vdisk
create partition primary
format
detach vdisk

```

Install [VirtualBox](http://virtualbox.org). Create a new virtual machine. Make
sure to choose "Use an existing virtual hard drive file". For the new virtual
machine choose
`Settings`
`Storage`
`IDE`
and add the ISO file. Start the virtual machine. Once its started you can export
a key (Shift + F10)

```
reg export hkcr\unknown C:\unknown.reg

```

Now power off the virtual machine and close VirtualBox Manager. Run the
following to mount the VHD, then the REG file will be available on the host
computer. Be careful not to run this too soon after closing VirtualBox. If you
reattach before the VirtualBox process terminates the file will not be saved.

```
automount scrub
attach vdisk

```

Final result

```
Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\unknown\shell]
@="openas"

```
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)