There are Bluetooth adapters that connect to your receiver. You would then need Bluetooth on your computer (if you do not already have it). Here is an example (there are many other brands/models):http://www.amazon.com/Logitech-Wireless-Speaker-Bluetooth-980-000540/dp/B004VM1T5S/ref=sr_1_3?ie=UTF8&qid=1369106663&sr=8-3&keywords=wifi+audio+adapter
Another option would be an "media adapter/device". The example that I know will work is either an AppleTV or an AirPort Express...but I suspect that there are other options (I believe a Roku would also be able to do stream audio from the likes of Windows Media Player, but I have not looked into setting up that functionality with my Roku device as it is connected to my secondary TV). By default with an AppleTV or AirPort Express, you can use iTunes to "stream" music to them from your iTunes library and thus to a receiver/speakers that are connected to them. If you want to use more than just music that you might have (or be able to get into) iTunes, then you can use Airfoil to stream ANY audio (say Pandora or Spotify, etc from a browser) from the computer to the AppleTV or AirPort Express.http://www.rogueamoeba.com/airfoil/windows/http://www.rogueamoeba.com/airfoil/mac/
I personally have an AppleTV connected to my receiver/speaker system in my family room and an AirPort Express (also used to extend my wireless network) into an old set of computer speakers in my living room upstairs. While I don't currently use Airfoil, I do regularly use them to stream my music from iTunes on my computer.
FWIW, it sounds like you would want an AirPort Express rather than an AppleTV. There is a mini-headphone style audio jack that you then connect to receiver using the 3.5mm to Red/White adapter cable. That same jack works as an optical audio port...so you could use an optical audio cable to your receiver if your receiver has an optical audio input jack...if you want digital audio.