One of the new features in Finder 7 is that it can substitute one application for another, assuming the second application can open the first one's documents. This is why the Finder will try to open TEXT files and PICT files in TeachText if it can't find the original application. You can add your own substitutions (ADAM: I personally have all TEXT files, Word files, and MacWrite files open in Nisus, which is much handier than keeping copies of everything else around.).
First, make a copy of the Finder, as ResEdit won't allow you to work on the currently active Finder. Launch ResEdit and open the copied Finder. Next double-click on the fmap resource. Now open resource # 17010. You will see a window with hexadecimal code on the left and garbled text on the right. The text on the right should read:
TEXTttxt PICTttxt 00000000
The last line is actually a line of rectangles, not zeros. Select the entire line of rectangles, and choose Copy from the Edit menu. Then, without deselecting the line of rectangles, choose Paste from the Edit menu. Nothing much should happen. Now choose Paste again, and you should end up with a second line of rectangles under the first, so that the whole thing looks like this:
TEXTttxt PICTttxt 00000000 00000000
Now, select the entire first line of rectangles, and type in the document type and creator that you want that document opened with. For example, if you want all MacWrite documents to be opened by Microsoft Word, you would type "WORDMSWD" so that when you are done, the text looks like this:
TEXTttxt PICTttxt WORDMSWD 00000000
Now close the fmap #17010 window, the fmap window, and the Finder window.
Click Yes when ResEdit asks you if you want to save changes. Quit ResEdit, move the currently active Finder out of the System Folder, and rename the "Finder copy" to "Finder." Reboot the machine, and from now on when you double click on a MacWrite document, the Finder will ask you if you would like to open it with Microsoft Word. Other combinations can be added to the Finder using the same method of replacing those eight rectangles (non-printing characters) with the four character type of the document and then the four character creator of the application you wish to use instead of the original one.