Need to make sure both sticks are the same type and even though it sounds obvious make sure it's properly seated in the slot. I used to get the blue screen of death every now and then. No idea why. Then one day thought I'm sick of only have 256mb of RAM and went and bought the same again. Opened up the PC and found that I already had 512 I'd just left a stick in there for months not properly in the slot and totally forgotten I had it. I have no idea to this day how I missed that. I'm supposed to know what I'm doing

I built this PC myself!