The new XP skin (all that Apple-like crap) is fancy and all, but it still runs on some HUGE graphics libraries. I really wish MS would switch to a gif or jpeg based OS GUI, but whatever.
Anyway, yeah, switching the Appearance settings to "Classic" helps a lot. Also, get rid of any animations for dialogs and menus unless you have a totally beefy card. This can be found and altered under the "Effects" button on the Appearances tab of the Display Properties dialog.
Also, don't use any animated cursors. And, depending on how beefy your graphics card is, don't use any heavy (big file size) background pics. In fact, if I'm on a sub-awsome computer, I always run with a no-picture black background.
Other than that, you can streamline your system with tweaks and other stuff, but you'll have to search around the web for that as I'm not writing it all!