I am trying to hook up front and rear and center speakers and subwoofer on to my PC. I have the cables plugged into the subwoofer like it says to do. Have gone in and changed my computer to 5.1, but only the front speakers work.

Usually, the audio source has to be channelized in order to get sound out of all the speakers without any special processing. Try a DVD movie or a video game. Also check your speaker documentation as some speaker systems have the capability or mode to synthetically expand 2 channel stereo input into 5.1 surround.

Otherwise, if you are playing a 5.1 source and only getting audio from the front speakers, the system isn't connected or configured correctly.