You can do that in Folder Options>File Types. Scroll to HTML Document and double click on it. In the big white actions box, double click on edit. In the new action box,Application used to perform action will be C:\windows\notepad.exe Press Ok Close Close
Or if you already have another app doing the editing and you want to add another editor do it this way.
Scroll to Html Doc and double click on it. Click the New Button under the actions box. In the new action which will appear>>>Edit in Notepad.
In the Application used to perform action
C:\windows\notepad.exe Press Ok Close Close