There are hell number of good features there in windows 7 taskbar [ via AskVG.com ] that you could wish to use in vista and xp taskbar, some of these features are like larger icons, no text of the application opened only icons are visible, easy grouping of opened windows and some other features like aero peak, taskbar thumbnails preview and jump list etc.
Sbar is a free small program which allows you to replace windows vista and xp taskbar which emulates the windows 7 interface and add all the functions which are there in windows 7 taskbar.
Credit: SBar" is a cool taskbar replacement for Windows XP and Vista which brings the same Superbar functionality. Thanks to "DimitarCC" of WinMatrix
Note: If you are looking for other way round and want to convert windows 7 taskbar like windows vista then you can refer this simple tutorial by cool HowToGeek