Windows 7 offers you location specific Aero themes based on the language and location you pick during installation. Here we will take a look at accessing other hidden themes from different countries.
When you first start the Windows 7 installation the first thing you will do is select a language, time and currency format. This is where Windows determines what themes you’re presented with by default.

From: http://tutorialsland.blogspot.com/2010/03/get-hidden-regional-themes-in-windows-7.html