The act of opening the outbox folder marks one of the emails as "read" which means it does not get sent. Single clicking on any email in the outbox will mark that item as read will do the same thing, even though there is no preview pane and the message is not actually being opened.
All add-ins have been disabled and the behavior persists.
Security software is Microsoft Security Essentials.
This is a critical issue as it is extremely easy for people to accidentally do this and then time-sensitive emails are not being sent out.
Outlook 2007, Windows 7 Professional, Home Premium, and XP Professional and Home Premium.