The sidebar has been leaked and it works in XP. Here are the instructions how to download and install it.

1. Download the Vista sidebar, and decompress it (it’s a RAR file).
2. Copy the decompressed folder into the Program Files directory (C:\Program Files\).
3. Now go to the Users folder, and under your username create a new folder named Parts (C:\Documents and Settings\x20 in my case), and copy the files from C:\Program Files\Windows Sidebar\Parts into this folder.
4. Download this patch and put it in the Windows Sidebar folder mentioned above, then run the file named Patch (and do as it says).
5. Next up, download XMLRW.DLL and place it into the Windows Sidebar folder. This file is part of Internet Explorer 7, so if you have that beta, just get it from your own IE7 folder.
6. Now just run the sidebar (C:\Program Files\Windows Sidebar\sidebar.exe) and click the plus sign to load the widgets!

Via [tipmonkies]