Lifehacker has found NiwradSoft Seven Remix XP, a theme pack that convincingly gives your computer a Windows 7 look and feel, complete with boot screen, desktop, and start menu. Of course, there's no integrated search box in the Start menu or folders, but NiwradSoft is giving you a theme pack, not a miracle.
From there, you can add additional Windows 7 desktop behavior, such as Aero Snap, Aero Shake, and Aero Peek. When you're done, daydream about switching to Windows 7. Go ahead and try -- after all, it's Friday.