This Vista theme has achieved a great balance between being dark but not depressing. I like the black and gray Start menu and taskbar. Even the transparency effect is dark which gives this theme a cool feel. I like the blue and green laser background better then the leaf one because it brings a great flow to the whole package.
If you want to use this theme on any version of Windows XP, you have to download the XP Theme Patcher.
Download: Vista UltimateDark Theme