For Outlook 2010:

1.  Click the View tab.

2. In the Layout section, click on Reading Pane.

3.Select Options from the pop-up menu that appears.
4.With the Reading Pane window open, uncheck Mark items as read when viewed in the Reading Pane and click ok.

For Outlook 2007:

To stop Outlook from marking messages as read when viewed in the reading pane, do the following:

1. Open Outlook
2. Click on Tools on the top menu bar
3. Select Options from the dropdown menu
4. Select the Other tab
5. Click the Reading Pane button
6. Unselect "Mark items as read when viewed in the Reading Pane"
7. Click OK
8. Click Apply, then OK again