It could be a hardware incompatibilty with the RAM u bought. I've gotten that once or twice before esp with mass produced (dell, compaq, hp, etc) computers. Check to see what are the recommended RAM manufacturers Micron or crucial is usually on that list so that'd be a safe bet as opposed to some generic brand. Since u said it works with a 128MB stick we are at least sure the slot itself is not malfunctioning, The fact that u also stated that u have a 512 in there says it should at least take another 512MB.