Vista Transformation Pack is a free software that replaces many resources in Windows XP/Windows Server 2003 and give them a look and feel of Windows Vista.

It can change such things as:

Boot screen
Welcome Screen / Logon Screen
New msstyles files (visual styles)
New desktop and file icons
New toolbar icons
Progress Dialogs
Sounds scheme
System Tray icons
New Wallpapers
Windows Media Player Skins
And much more