Some times Windows 7 has a Themes crash. When this happens, you will not be able to change themes. It also crashes the Aero look, which gives the old Classic look to the windows and buttons and you lose some of the Aero functions.
To correct this problem, click on Start, type services.msc, and press Enter.
This will open the Services dialog. Scroll down to the Themes item. Double click on Themes.
This will open the Themes Properties dialog. Make sure the Startup type is Automatic. Click on the Stop button and then on the Start button.
This should correct the problem. You should notice Windows changing to Aero.