You should have been prompted to download a patch for windows that allows it to see your 500gb drive. I remember something like this a while back when I built my first rig with a 200gb hard drive. It only recognized 137gb of it and when I first started windows it came up with a message saying it detected a drive larger than 137gb and then gave me a link to download a patch from microsoft, which I did and it worked fine. The XP disc I have isn't even SP1 though.