And remember, a quad core cpu isn't going to speed up windows XP.
May a little bit and might help with multi tasking but it isn't going to be a life changer as far as OS performance goes. At least thats what I figure you are talking about.
You'd be better off with a re-format if your windows is running slow. Also, get a 2nd GB of ram in there it will make a world of difference. 2GB is golden for XP.