do this...power the desktop off. pull power from the wall for safety. open the case and pull both sticks of ram. put the ram back into both slots. make sure there no dust or bent pins. turn on the pc. see if with cpu-z both memory stick are read. if only one stick is swap memory slots. if there no change take one simm put it in each slots make sure it read same for the other stick of ram. if both sticks works it could be a mb or cpu memory controler failure. before you pull the mb. pick up a new set of ram. most 8g ddr3 ram is cheap. if you have a local computer store pick up a set try the new set if the error still there return the ram.