If you're looking for an Expose emulator to port that cool app-switching feature from the Mac to Windows, DExposE2 is probably about as close as you can get to a direct copy for either Windows XP or Vista. It gives them three choices for jumping between open programs, and it doesn't slurp up more RAM than it should.
Using the F9 key, you'll be able to minimize your applications to fit on the screen, then choose one to bring to the front by hitting Tab or left-clicking your mouse. F10 highlights only the active windows belonging to the application currently running in the foreground. F11 reveals the desktop by shunting all active windows off to the right side of your screen, visible only as a thin and unidentifiable bar.
DExposE2 lets you set up other hot keys or move your mouse to the corner of the screen to activate, and there's also a Windows Taskbar icon you can use to access it's features--but the hot keys are more than adequate. Other customizations include setting a background image, determining the behavior of previously minimized windows, and customizing program labels. Three levels of graphical quality should allow those with less RAM to scale the program to their needs. Although some crashes have been reported, DExposE2 worked flawlessly for me.