For older laptops/computers, some of the shinier features might prove burdensome on your browser (like 3D dice or dynamic lighting for example) or really big maps with lots of images on it. Fortunately, the big flashy stuff is often something you can toggle on and off as either GM or Player. If you're the GM, you can play around and see how much your machine can handle and then plan your game construction around minimal processing power. Like Apsara said, there's nothing to install outside of keeping your Java, Flash and browser relatively up-to-date.
