Vista Transformation Pack: Transform Windows XP & Windows 2000 to Windows Vista for Free
Aptly called the "Vista Transformation Pack", this piece of software will instantly transform Windows XP or a Windows 2000 installation to look and feel just like Windows Vista complete with the "aero" buttons, desktop wallpaper, Vista Dock and Sidebar with Microsoft Gadgets, Windows Vista Welcome screen, Sound Scheme, new Vista icons, Theme and Media Player skins and even a new Windows Vista bootscreen, which replaces the "Starting Windows XP' screen with Vista logo and message.
Infact after the installation the system so perfectly mimics Windows Vista that it's difficult to sometimes spot the original from the real one. Installation is pretty much a one click affair but make sure you have about 512MB of RAM if you want all the visual effects of Windows Vista on your Windows XP!
Download it from here and experience the 'Vista' experience without the bugs ;)