Source: comments in How can I get Outlook 2003 to default the reading pane off, and to stop grouping?

The steps outlined here will turn off AutoPreview, Reading Panes, and all Group-By settings. Keep in mind that you can use this method to apply other changes based on your personal preferences.

1. Create a Shortcut for OUTLOOK.EXE and place it on the Desktop.
2. Edit the Shortcut properties and add the /cleanviews switch to the target line so it looks something like this: "C:\\Program Files\\Microsoft Office\\OFFICE11\\OUTLOOK.EXE" /cleanviews
3. Start Outlook using the shortcut.
4. Select the Inbox folder.
5. Navigate the menus to View | Arrange By | Current Views | Define View.
6. Select Messages and click the Modify button.
7. Click the Group By button and uncheck Automatically Group.
8. Click OK.
9. Click the Other Settings button and set Reading Pane to Off.
10. Click OK, twice.
11. Now this is the tricky part! Click on any other view listed in the dialog box OTHER THAN Messages and click Apply (Last 7 days, for example).
12. Navigate the menus again to View | Arrange By | Current Views | Define View.
13. Now choose Messages and click Apply.

Your new custom view will now be applied to all of your Outlook folders. However, this will not happen instantly. It will take some time for Outlook to work its way through your folders. If you have numerous folders and a deep hierarchy, you may have to click on the intended folder, then click another folder, and then click the intended folder again to get your view to display correctly.

Not exactly an elegant solution to be sure, but it has worked in all of the testing we have done at TechRepublic. If you encounter problems, please share it with us in the article discussion; perhaps we can figure out a better solution.