Assuming Jim is right its your bios thats not setup for 64bit. It will run 64bit windows ok, but the memory controller found in your NB located on your motherboard can't. Its a 32bit only controller? Something newer then a 945 will probably allow you to use all 4GB. I vote that you don't worry about it. The performance difference between 3.25 and 4.0GBs is nothing.