It looks to me like the choice is controled by a registry key:
HKEY_CLASSES_ROOT\\.htm\\OpenWithList\\Notepad\\shell\\edit\\command

You might play with the registry and insert your prefered editor.
Alternately you might just remove the section from Notepad down, and let the system ask you what you want to use.

If you do this be sure to save a copy of the registry hive first.

Hugh