Cut and paste the following script into notepadunknown.reg and run it.

REGEDIT4
BLANK LINE GOES HERE
[HKEY_CLASSES_ROOT\Unknown\shell\Open\Command] @="notepad.exe %1"
BLANK LINE GOES HERE

This should work for all versions of Windows. See .REG files for background information on .REG files.

Related tips:

• Set Explorer to open unknown files with Notepad
• Configure Notepad to automatically insert the date / time
• Make Notepad Wordwrap