I have a friend who owns a HP Pavilion DC2037US laptop running on an Intel Centrino Mobile and 1 GB of PC2-4200 DDR2 So-Dimm memory (2 dimms, 512 mb per slot) and Win XP. I got a hold of two 1 GB PC2-6400 DDR2 So-Dimm memory modules, tried installing them into this computer and the thing will not boot after the install of new modules. The computer just beeps once very loudly and shows a black screen. I thought PC2-6400 should be backwards compatible with PC2-4200 and clocks down accordingly. I've done my research and know this computer will accept a maximum of 2 GB and each memeory slot will read up to 1 GB. I've also tried installing just one new module and keeping one 512mb in the other slot. The computer turns on but only reads 512mb of ram. I've upgraded ram in my Compaq laptop that also originally came with PC2-4200 DDR2 and replaced it with PC2-5300 DDR2 and everything works fine. What's the deal here?