Isn't that the whole point of a new version - to replace the old one?
But anyway, they're releasing some aspects of Vista for XP users - IE 7, the file system, DirectX, Avalon (I think, or it might be Aero).
Edit: Btw, Windows releases tend to get slower and slower (although technology gets faster and faster so no-one really notices) - just try running Windows 95 on your PC (if you can) and watch how fast it goes.