You can preview Outlook email messages without opening them by using the Reading Pane.
To control if you want the Reading Pane open, and where you want it to appear in Outlook...
1. On Outlook's menu bar, click "View"
2. Hover your mouse pointer over "Reading Pane"
3. Click on your preference.
Repeat for any additional folders for which you want to customize the Reading Pane.
More info on customizing Outlook 2007's Reading Pane can be found at this Microsoft Office Outlook page.