In outlook 2013 ,
when you reply a message you have to enter your message in the bottom preview pane.
There are two options, "Pop out" and "discard".
ou can disable composing in the reading pane using a reg key - the buttons will open a new window.
-> Options-> Mail-> Replies and Forwards-> Use inline replies when replying or forwarding
In RTM this option is :
File-> Options-> Mail-> Replies and Forwards-> Open replies and forwards in a new window