the problem here is not file association, unless you want to associate Notepad as the default program to open all HTML files (rather than your web browser).
i recommend this method instead:
1.
Enter the program's executable name (notepad.exe) as a subkey of these two registry key's shown below:
[HKEY_CLASSES_ROOT\.htm\OpenWithList\notepad.exe]
[HKEY_CLASSES_ROOT\.html\OpenWithList\notepad.exe]
2.
And add an "edit" subkey here:
[HKEY_CLASSES_ROOT\Applications\notepad.exe]
[HKEY_CLASSES_ROOT\Applications\notepad.exe\shell]
[HKEY_CLASSES_ROOT\Applications\notepad.exe\shell\edit]
[HKEY_CLASSES_ROOT\Applications\notepad.exe\shell\edit\command]
Default = C:\WINDOWS\notepad.exe "%1"
3.
To change the default editor, replace the "Default" entry in this registry key
with the command line for your prefered editor:
[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Default HTML Editor]
[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Default HTML Editor\shell]
[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Default HTML Editor\shell\edit]
[HKEY_CURRENT_USER\Software\Microsoft\Internet Explorer\Default HTML Editor\shell\edit\command]
Default = C:\WINDOWS\notepad.exe "%1"