Find a .dat file in Explorer.
- Use shift-right click to call Open with ... dialog. Make it Notepad again, and check the always-open-with-box.
- In the future, if you occasionally want to use another program than the default Notepad, use shift-right click to get the Open with ... dialog

To totally remove a default option, in Explorer go to Tools>Folder Options>File Types, search the extension, select open and click remove.

Hope this helps,