Ubuntu Skin Pack is a complete Unity-like skin for Windows 7 or XP. There's no Dash, but almost everything else is covered: icons (Humanity for the system and Faenza for the launcher), the Unity launcher, boot and login screen, the Ubuntu font as well as the Ubuntu 11.04 wallpapers.
The Ubuntu Skin Pack DeviantArt page even links to a small utility that emulates the famous Compiz cube (for more such tools, also visit the Maverick for Win 7 DeviantArt page):
But of course, being a different OS, the skin pack has a few issues. For instance, the window controls are out of place in Windows 7, this being an old issue that occurs in all such skins. Also, there are no global menu or window controls on the top panel - I'm not sure if that's intended or it's just not possible under Windows.
I don't use Windows so I haven't tested Ubuntu Skin Pack for Windows but if you try it, let us know what you think!
Download Ubuntu Skin Pack: Windows 7 32bit | 64bit, Windows XP - the installer contains adware (browser toolbar) (make a backup before installing it!)
Thanks to Pedro Inácio for the tip!