I installed a Vista like sidebar in XP a while ago, but quickly got bored of it and concluded that it didn't really give me anything I needed and instead just used up some of the screen.
If I want a calendar there is one built into Outlook. If I want a clock there is one in the corner of the taskbar. If I want a calculator i can hit the calculator key on my keyboard and use the standard Windows one. Notes? Outlook again. The sidebar is pointless in my view.
And don't get me started on the transparent windows in Vista. Pretty much everything added into Vista is polish and effects, and nothing much is actually a new feature. About the only thing I do like is the new directory navigation and searching which works in a similar way to directory opus.