You're basically wanting a "Picture-in-Picture" effect. Google turned up a few results but it seems like Movie Maker can't do it easily.
http://www.windowsmoviemakers.net/Pa..._Tutorial.aspx is the only thing I saw explaining how to do it in MM2.
There's some other programs that can do it easier, like Premiere, but I'll let you search for yourself now that you know what to search for
