Although Vista is new, and fast, sometimes it can seem a bit bogged down. Too many programs and not enough resources could be your problem. This could be due to having Vista Aero enabled.
Aero is a resource consuming hog. Vista is a resource hog. Vista Aero in particular eats of a ton of resources. Aero is a group of features with Vista, such as visual effects, including helpful animations, transparent glass menu bars, and live thumbnail previews of open programs. Aero is setup as the default theme for Vista.
Disabling it frees up allot of resources and you should notice a difference in the system after you disable it. You will still have the Vista look, but not the resource hogging effects. Once you disable Aero you will lose the 3-D flip option for your desktop so please take that into consideration before you decide to disable Aero. Its not a definite necessity but some people like it.
If you want to learn how to disable Aero, you can visit the boost vista performance blog. This will definitely speed up vista and make you a happy camper again.