First of all, make sure your Outlook Express security settings are correctly set to the recommended safe options. Click on Tools
, and make sure "Restricted sites zone"
, "Warn me when other applications try to send mail as me"
, and "Do not allow attachments to be saved or opened that could potentially be a virus"
Secondly make sure you don't have the Preview Pane enabled. The preview pane basically opens the message when you single-click on it and you do NOT want that. To disable it, go to View, Layout, and uncheck "Show preview pane".