You did go through the receiver setup?
That tells the receiver what outputs you have speakers connected to, which can handle bass, how far from you each is, and that each plays at the same volume on test in your seating position.
If your PC is outputting in digital surround sound the receiver will indicate this on the front panel. Set the surround mode to auto so the receiver can switch to the correct one. If the PC is in stereo then you need to select an analog surround mode designed to give you surround with a stereo input.