Windows itself doesn't set that association, Office does when it's installed. The reason, I suppose, is that Microsoft intended to make it so that editing HTML files was easier for 'regular people', and Word can (sort of) achieve that.
To change the 'Edit' association, you'll need to go into Windows's file types settings:
- Open any folder in Explorer (My Computer, for example)
- Go to Tools > Folder Options
- Go to the File Types tab
- Scroll to and then highlight HTM and/or HTML (you can just tap H on the keyboard probably)
- Click the Advanced button
- In the Edit File Type window, you will see 'Edit' listed under actions; make sure this is highlighted, and then click the Edit button
(added). If 'edit' is not listed, then add it via the new button. 'edit', no caps.
- In the action editing window, first untick 'Use DDE'
- Then, in the 'Application used to perform action' field, change the application path to the application you want to use. Leave the
%1 at the end alone. So, for example, to change it to Notepad, you can do:
- Click OK, OK, OK