By default, Outlook is configured to mark a message read a few seconds after the message is selected. This can result in messages accidentally marked as read.
You have the choice of automatically marking messages read after a few seconds, marking messages read when you select another message or never marking the messages read (by having both options unticked.)
My preference is to never automatically mark messages read. They will be marked as read if I double click to open or Reply/Forward. I use Ctrl+Q to mark other items read as I read them. (Use Ctrl+U to mark messages Unread)
Reading Pane Options Tutorial
In Outlook 2010 and newer, the Reading Pane options are accessible from the View menu, Reading Pane, Options.
In Outlook 2007 and older, change the settings or disable this option in Tools, Options, Other tab, Reading pane options.