You can enjoy windows 7 on your Windows Vista and XP with Windows 7 Transformation pack, Windows 7 Taskbar Sbar, Windows 7 Screensaver for Xp. Now here is way to get Windows 7 Sidebar in Windows XP & Vista.
Thoosje Windows 7 Sidebar is a Windows 7 Style sidebar for Microsoft Windows XP and Vista that has gadgets like media players, Sticky´s, Calculators, Google search, Yahoo search, and a large number of more search engines. It has also Vista sidebar gadgets like weather and system uptime and lots more useful gadgets.
Thoosje Windows 7 Sidebar in Action
Image Credit & Source: Jkwebtalks