Richo,
What do the instructions say on your Creative sound card in regards to hooking up a home receiver? Logically, I would think the signal from your PC soundcard's rca out would be input to the Yamaha's aux. in, and then controlled by the Yamaha. Is that how the two units are hooked up? That could explain the two speaker problem. If the soundcard is putting out a digital signal, you may need a digital out from the pc to a digital in on the receiver (optical or coax). If you are still having sound issues with all 5 speakers, it could be your material is not true 5.1 sound. Did you try a 5.1 dolby digital dvd in your pc, and is your Creative sound card a 5.1 card? If all else fails, you can still run two channels out of the pc into the receiver and select a matrix dsp mode to give you matrixed 5.1 sound.