With a few modifications the vista sidebar has been hacked enough to get it to work with windows XP. Here are the directions.
You need the attached file . It includes all the files you need for this hack.
1)Download the file and unrar it somewhere. Copy the folder "Windows Sidebar" into your Program Files folder. If C is your default drive, the folder should end up here: C:\Program Files\Windows Sidebar
2)Copy the files from folder "preview_build_01" into C:\Program Files\Windows Sidebar. Click on the patch.exe and press Start. In the new window press OK.
3)Some people reported issues with the sidebar needing MFC71.dll . If you need this dll, copy it from the folder MFC71.dll into C:\WINDOWS\system32 folder.
4)Run sidebar.exe from folder C:\Program Files\Windows Sidebar.
-right click on sidebar.exe from folder C:\Program Files\Windows Sidebar and choose copy.
-Go to folder C:Documents and Settings\Your_Name\Start Menu\Programs\Startup, right click somewhere in this folder and choose paste shortcut.