Is the memory failing in one memory slot?, when you boot up do you get any errors ?, is the memory of the same type / make?
does the motherboard do parity checking on the memory and if so is the system set up for it?
If the only time you get the error is when you run Norton my advice would be carry on as if nothing is wrong, but if it really bothers you maybe its worth spending £15 or so on a new 128Mb memory module and putting it in and retrying the test