The latest update of Rogue Amoeba’s Airfoil, an application which allows users to send any audio to an AirPort Express, and other Macs, now includes AppleTV support and also also Airfoil Speakers for Windows.
RA has also fixed DVD playback, improved performance on low-end machines and improved compatibility with Front Row. Airfoil requires Mac OS X 10.4 and costs $25. You can download a demo of it right here.