This articles provides the steps required to disable the feature that includes messages in replies when using Windows Live Mail 2011.
Including messages while replying is useful when you want to keep a reference to the original email from the sender. But when the conversation grows, it will become manageable to keep track of things and you may have to manually delete the previous content when replying. Windows Live Mail 2011 provides option to the users to automatically stop including messages in replies.
Click the File menu, select Mail under Options menu.
In the Options window, click the Send tab and navigate to Sending section.
Now un mark the check box with label as Include message in reply and click Ok button. This would turn off the feature that includes the messages in replies.