Optical audio from PC to reciever

I managed to get the Onkyo HT-SR800 for a really good price. I bought it to use with my computer to play movies and games in 5.1. I hooked it up with an optical cable from my motherboards optical out to optical input 1 on my reciever.

The computer says it is outputing through optical and I set the reciever to play from the optical in and I get no sound.

The speakers are set up right, when I hook the computer up with a standard stereo plug (whatever they are called, headphone jack to red and white haha) I get sound on my front two speakers and sub.

What could be going wrong outputing through optical? Even the test in vista doesnt make any sound come out

I am running Vista Ultimate, 3.6ghz Q6600 (quad core), 4 gigs of ram and an 8800GTS 512mb on an Asus P5k-e