Seven Transformation Pack
It's fairly easy to make Windows Vista look like Windows 7. However, Windows XP users have fewer options at their disposal. But now they have the Seven Transformation Pack, a collection of UI tweaks--menus, icons, fonts, buttons, etc.--designed to make XP look and feel just like Windows 7. And guess what: It works as advertised. The before-and-after difference is pretty amazing, right down to the search-enhanced Start menu and Windows 7-style floating gadgets.
Before downloading this fab freebie, make sure your system has XP Service Pack 3 installed. I'd also advise creating a restore point in XP's System Restore tool before running the installer, as the Seven Transformation Pack makes some pretty high-level changes to your system. When it's done, reboot, then be prepared to witness the minor miracle of Windows XP transformed to Windows 7.
Note: This link takes you to the vendor's site, where you can download the latest version of the software.