SevenMizer let you transform Windows XP, MCE or Server 2003 in to Windows 7. All it does is change the taskbar, mimic the Start Menu, Icons on the desktop and in Windows Explorer’re also adapted to Windows 7 style. The installer let you decide which files’ll be modified and which not. However if the result of the installation should not assure to you, you’ve the possibility to get back the old system by an un-installation. If newer files’re already present in the system by updates, this’s recognized by un-installer and only modified files’ll be replaced. Download
If you want to add some Windows 7 functionality to XP, third-party software like: AeroPeek for instance achieves almost the same effect on XP as the Win7. AeroSnap fills the shoes of the function with the same name under Windows 7, while ShockAero provides Flip 3D effect. ViGlance gives access to much acclaimed taskbar (preview and task-switching).