I have to admit, switching between Windows XP and Classic Windows themes isn't the most exciting way to customize your desktop. However, you can add more themes that will give you a variety of new looks for Windows.
There are many sites you can visit to buy themes and even download free themes. PC World magazine has a good selection of free themes at http://www.pcworld.com/downloads/.
Microsoft will sell you new themes as part of its $30 Microsoft Plus Pack. To get to Microsoft's Web page, right click your desktop to open the Display Properties dialog box, click the Themes tab, and select More Themes online.