To get 5.1 channels over a digital S/PDIF connection, either optical or coax, the source player (your PC in this case) must provide the audio encoded as either Dolby Digital or DTS. If the computer is outputting the audio as PCM, S/PDIF can only transport a two-channel (stereo) signal.
To transport multichannel PCM, you have to use the computer's HDMI connection, which can provide up to 7.1 channels. You also have to install the latest drivers from Nvidia and enable the HDMI audio output in the audio "Playback devices" menu.
Connect the computer's HDMI output to one of the two HDMI inputs on the back of your HTIB. Connect the HTIB's HDMI output to the TV. Power up the TV and then the HTIB and finally boot the computer. Both TV and HTIB have to be powered up for the HDMI output to appear in the computer's audio "Playback devices" menu.