Vista is a hog when it comes to system resources.
If I could make another suggestion. There's an utility app called Advanced SystemCare put out by IObit that does a lot of neat stuff for keeping your computer clean and running more efficiently like getting rid of junk and defragging. All sorts of tools in one app. It also has a power booster in it. It comes up on the right-click menu when you run SL and I've found it to be quite helpful. Won't make it perfect, but every little bit helps when it comes to running SL I find. And there's a free version!