Improving the Windows XP User Interface?

Pimpin' Up Windows asks: "Many of us are forced to live with Windows XP for our day-to-day computing needs - at work, home or school - and longingly look to the not only beautiful, but functional and efficient, Mac OS X 'Aqua' user interface. Apart from just themes, what would be Slashdot reader's suggestions for improving the user interface of XP? What changes, add-ons and other improvements could further enhance its usability?"