Windows 7 offers you location specific Aero themes based on the language and location you pick during installation. To access the other international themes click on the Start button and enter 'C:\Windows\Globalization\MCT' into the Search box.
Make sure to change C:\ if your drive letter is different. In the MCT folder you will find additional themes for Canada, Australia, South Africa, and Great Britain. Open the Theme folder for any theme and double-click on the .Theme file to enable it. All themes you choose will now be available in the My Themes section.