My back yard has been taken over by this:

http://members.socket.net/~jfalken/img/weed1.jpg

http://members.socket.net/~jfalken/img/weed2.jpg

It came from a vacant lot next to mine. The builder cut it all down,
but not before it literally filled my back yard. Any idea what it is
and/or how to kill it without killing the grass. I just had the house
built last year and the grass was starting to grow. Then the weed
came and while my grass is still down there, it doesn't have a chance.
Thanks for your help.