In Windows 98 (XP may be similar) you can open Windows Explorer, click the 'View | Folder Options' menu, switch to the 'File Types' tab, and then find the 'DAT File' entry to remove or change it.
There are some file management tools listed on
this page, one of which is for managing file associations. I have not tried it, but it should be able to delete associations.
You may want to add a shortcut to Notepad in your Windows\SendTo folder (so you can use the right click 'Send To' option to open files, or leave a shortcut on your desktop (to drag and drop files onto). There are registry edits (and probably programs, search Google) that will let you add an 'open with Notepad' option directly to the right click context menu, which would save you a couple steps and avoid accidental associations.