all USE flags are default except added sse mmx 3dnow can't even get it to switch out of fullscreen mode. doesn't matter which video output i use, always starts full screen.
You need to read the man page for mplayer. You can pass it command line switches to start as a window. Additionally I think /etc/mplayer.conf (or something similar) is available for setting system wide config options, otherwise you should have a $HOME/.mplayer directory with config settings in it.
fs=no # Enlarges movie window to your desktop's size. # Used by drivers: all either in your /etc/mplayer.conf file or the conf file in your ~/.mplayer/config file.