Telestream Wirecast can do it.
Download Wirecast (trial watermarked until purchased) and then download the free Desktop Presenter add on.
You can point DTP to the entire desktop or just a specific window you have open. If you have webcam or DV cam hooked up you can even do Picture in Picture if you like.
You can also send from one computer to another. Let's say you set up your stream from one desktop, you can open DTP on another computer on your same local network and take that desktop as a source.