Get the Vista sidebar in XP
Installing the sidebar consists of installing three packages: Alky for Applications, Windows Sidebar Installer, and Gadget Extractor. Alky for Applications is where the magic happens allowing Vista-specific software to run under XP. An interesting side-effect of installing the Windows Sidebar Installer is that the Vista-specific fonts are added to your XP installation.
Because this is an XP-compatible version of the actual Vista sidebar rather than a copy, all of the widgets that you could install in Vista will work on XP.