The device you are looking for is the reasonably priced
Sewell Digital to Analog 5.1 Dts/ac-3 Decoder Surround Sound (Toslink/coax to Rca)
If you need pc 5.1 outputs, you can easily attach three of these to the sewell
http://www.amazon.com/Cables-Go-Stereo-Female-Y-Cable/dp/B0002GUBIS/ref=sr_1_11?%20s=electronics&ie=UTF8&qid=1318904820&sr=1-11