There is a workaround to get back the Windows Vista Sidebar in Windows 7. Follow these steps:
1. Close the Windows 7 sidebar by killing the sidebar.exe in the task manager
2. Download this zip file: Win7-Sidebar-Fix.zip
3. Extract the zip content to your Windows Sidebar folder C:\Program Files\Windows Sidebar (overwrite existing files)
4. Run the sidebar.exe from C:\Program Files\Windows Vista Sidebar
5. Add your gadgets and have fun with the Windows Vista Sidebar!