You're my hero of the day!!
When I opened your link, at first glance I thought it was over my head but I decided to open Regedit and look for Notepad. It lead to me solving the problem.
When the problem existed this is what the registry looked like:
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Unknown]
"AlwaysShowExt"=""
"QueryClassStore"=""
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Unknown\shell]
@="!8OpenInNotepad"
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Unknown\shell\!8OpenInNotepad]
@="Open in Notepad"
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Unknown\shell\!8OpenInNotepad\command]
@="\"notepad.exe\" %1"
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Unknown\shell\openas]
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Unknown\shell\openas\command]
@=hex(2):25,00,53,00,79,00,73,00,74,00,65,00,6d,00,52,00,6f,00,6f,00,74,00, 25,00,5c,00,73,00,79,00,73,00,74,00,65,00,6d,00,33,00,32,00,5c,00,72,00,75,00, 6e,00,64,00,6c,00,6c,00,33,00,32,00,2e,00,65,00,78,00,65,00,20,00,25,00,53, 00,79,00,73,00,74,00,65,00,6d,00,52,00,6f,00,6f,00,74,00,25,00,5c,00,73,00, 79,00,73,00,74,00,65,00,6d,00,33,00,32,00,5c,00,73,00,68,00,65,00,6c,00,6c, 00,33,00,32,00,2e,00,64,00,6c,00,6c,00,2c,00,4f,00,70,00,65,00,6e,00,41,00, 73,00,5f,00,52,00,75,00,6e,00,44,00,4c,00,4c,00,20,00,25,00,31,00,00,00
With the problem solved it looks like this:
Windows Registry Editor Version 5.00
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Unknown]
"AlwaysShowExt"=""
"QueryClassStore"=""
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Unknown\shell]
@="openas"
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Unknown\shell\openas]
[HKEY_LOCAL_MACHINE\SOFTWARE\Classes\Unknown\shell\openas\command]
@=hex(2):25,00,53,00,79,00,73,00,74,00,65,00,6d,00,52,00,6f,00,6f,00,74,00, 25,00,5c,00,73,00,79,00,73,00,74,00,65,00,6d,00,33,00,32,00,5c,00,72,00,75,00, 6e,00,64,00,6c,00,6c,00,33,00,32,00,2e,00,65,00,78,00,65,00,20,00,25,00,53, 00,79,00,73,00,74,00,65,00,6d,00,52,00,6f,00,6f,00,74,00,25,00,5c,00,73,00, 79,00,73,00,74,00,65,00,6d,00,33,00,32,00,5c,00,73,00,68,00,65,00,6c,00,6c, 00,33,00,32,00,2e,00,64,00,6c,00,6c,00,2c,00,4f,00,70,00,65,00,6e,00,41,00, 73,00,5f,00,52,00,75,00,6e,00,44,00,4c,00,4c,00,20,00,25,00,31,00,00,00
Once I'd located the relevant entry in the registry and tested the effect of deleting each of the keys it was easy, I just looked in the registry on another computer and then exported/imported the "unknown" key and the problem disappeared.
Thank you very much.