Is there software available that'll make a Windows PC running XP appear as an AirPlay destination in my Mac's iTunes? I'd like to be able to stream audio from my Macs to a Windows machine I use as my home theater PC that's connected to my stereo.
Reflector 2 allows you to AirPlay audio and video from iOS to Windows. It claims to support iOS 9 now also (note I personally haven't tested the iOS 9 support yet).
Reflector installs an app that acts as an AirPlay and Google Cast receiver. The app is available for Windows, Mac, Android, or Amazon Fire. Since Reflector supports receiving both AirPlay and Google Cast it can receive audio/video from iOS or Android.
It's from Squirrels, the same company that makes the excellent AirParrot app (which allows you send AirPlay audio and video from Windows and Mac to AirPlay and Google Cast).
If you only want to stream audio, try Airfoil Speakers, part of Airfoil for Windows
Here is a fast and simple open source program that works very well: http://sourceforge.net/projects/shairport4w/
Looks like AirMedia Player for Windows is made to let you do this from at least an iOS device, probably iTunes as well.
The WiFi2HiFi app is what you are looking for. It can stream audio from any computer program to the iPhone: http://www.wifi2hifi.com
Use airparrot to stream your windows screen to ur apple tv. http://airparrot.com