LiteStep It completely changes the Windows GUI and is somewhat difficult to setup -- but incredibly useful.
I've been using it for over 3 years now; I am way more productive when I use a PC with it installed. There are hundreds of custom GUIs you can download, I use a Gnome theme. The things I find most useful are 1) launching apps with keyboard shortcuts and 2) the virtual desktop interface which simulates 4 desktop spaces/screens that I can quickly switch back and forth between. I don't know what I'd do without it.