The links below are some of the add ons and enhancements I use with XP. At the present I am using all them except the Transformation Pack
IE 7 Pro - IE 7 add on
IE7Pro - The Ultimate Add-On for Internet Explorer
Vista Sidebar ( Longhorn )- Has many panels for date, time, weather, quick launch icons, system performance, can be docked and/or hidden.
Desktop Sidebar Home Page
Vista Transformation Pack
Vista Transformation Pack Win Vista
Vista Start Menu - Convert the XP Start Menu to Vista Start Menu
Vista Start Menu – Convenient alternative to Start menu