It would be best to connect the PC directly to the television. I'm not sure what the problem is, but it seems to be interference being sent by the PC if it's powered on and it's detected even if it's powered off. That may be fixed by a setting, or a driver update, or not.

That said, porting the HDMI from the PC to the television for video would be recommended, followed by connecting the audio to the receiver, either through an optical cable or an RCA cable, depending on what audio source is available on the PC. There's not a setting on the HT system that would be causing that, that I can think of.

The only remedy that might be related to the HT system would be the Anynet+ feature, which you can turn off in the HT system and the TV. If you'd like to try that, it might solve the issue. If not, it's likely something in the video card or something that I'd be unable to diagnose.