AirParrot (64-bit) lets you AirPlay your PC's screen to a second or third generation AppleTV. What you see on your PC's screen will appear on the AppleTV, wirelessly. It can even let you extend your desktop to the other device if you need a little more space. You're also able to optimize your video quality so you can find the right data rate for your network.
Version 1.1.6 fixed issues where the selected device in the menu could be incorrect if the device list changes while connecting and bug introduced in 1.1.5 where a congested Bonjour network may timeout prematurely.