I would like to mirror my Windows PC to my Apple TV via AirPlay. I've found a few options which enable Windows Media Player to receive video via AirPlay. I've also found an application named Airfoil which allows you to stream audio from Windows. I know that the beta of Mountain Lion supports AirPlay mirroring, but is there a Windows option?
I use a program called AirParrot to mirror my Windows PC to my Apple TV (third generation). There is also a version that works on Macs.
Note that the ability to mirror a Mac to an AppleTV is a new feature of OS X Mountain Lion.
You can achieve this indirectly if you have an iPad.
Install remote control software on the iPad (e.g. Teamviewer) that controls your PC and then use airplay mirroring to display the iPad's screen onto your apple tv.
How effective this is will depend upon the type of content you wish to mirror, video is not going to work but static content may be OK.