How do you get 5.1 sound working over HDMI with an a/v receiver?

If I plug my pc into the receiver then the receivers' output to my tv, it detects my tv as the device and says "nope, only 2 channels for you". If I extend my desktop (second hdmi to receiver with primary display output to tv) it detects that 6 channels are available, but the windows sound test, and anything else, doesn't work. For the record my receiver only has PCM, LPCM, dolby digital and DTS. Game consoles work fine.