Yeah I have already tried that and much more without any luck.
I have nothing else using 5.1 over HDMI as my other consoles have an optical out port.
I have read that this problem is pretty common with Windows 7, and that it might be a problem with handshaking, etc. but I am not sure how true that is.
The only other thing I can think of is that my tv isn't able to properly push the audio out to my receiver. It is an older(3 yr old Bravia) tv, but I'm not really sure about it's capabilities with things like this.
I am thinking of just getting a sound card with an optical out. I can't stand going back to 2 channel fake surround when everything else about my PC is so damn awesome.