Important: This procedure will not work in Windows 7 Starter, since it does not offer Aero support
Step 1: Video card driver installation
If you haven't got a discrete (non-integrated) GPU, you can just skip to step 2.
First you have to install your graphics card's driver. It will come in a CD inside the box. Insert it in your optical drive, then run the file.
Step 2: Windows Experience Index calcullation
If you've already got your Windows Experience Index calculated and up-to-date you can skip to step 3.
After you've installed the driver, go to the control panel and and then to "Performance information and tools". After that click "Calculate Windows Experience Index" (if it's not calculated yet) or "Refresh Now" (if calculated, but outdated).
Step 3: Configuring Aero
Click the Start button and type "aero". Then click on “Find and fix problems with transparency and other visual effects”.
A window will appear, keep clicking next.
The troubleshooter will then try to detect any problems, analyzing a number of components required to activate or turn on Aero, such as video memory, Desktop Windows Manager (DWM) service, color, themes, etc. and power setting. When finished reviewing your machine, the troubleshooter will attempt to fix error related to Aero.
Step 4: configuring transparency and color
Go to your desktop, right click anywhere and then on "Personalize".
A window will appear. Then click any of the themes under the "Aero Themes" section. To configure window transparency and color click "Window Color" at the bottom.
Here you can adjust the color and the transparency, as it is shown in the image below.
If you have any doubt or think there's something wrong in the tutorial please contact me.