If you already have Object Desktop you can do this. This week myself, Steve from TweakVista, Nate from Betanews, Bink from Bink.nu (who missed his plane back, tsk tsk), and several others were visiting Microsoft to get some of the latest news on Windows Vista.
While there, we put together a little joke using WindowBlinds + DesktopX + IconPackager + IconX in order to make a Windows XP machine look somewhat like Windows Vista. Using DesktopX we whipped up a fake build number, IconX to get the most of the Apogee icons, IconPackger to apply the icons, WindowBlinds to apply the VistaXP skin (skin not available here as Microsoft requested its removal).
Here's one of the screenshots we came up with. You can imagine the setting that all this took place in - a couple dozen techies in a room all communicating to each other with IM in the same room.

The original prank was put up on Bink.nu (see link below)