I take it you mean you already have 256Mb of memory in your machine. First make sure that any new memory is compatible ie: PC100 or PC133. The amount of memory you can put in is governed by your Motherboard and not by Win 98. Having said that it would be unusual for a board not to support a total of 512Mb. Check the memory is not faulty by only inserting the new memory by itself first. Lastly make sure any memory is properly seated in the Board.