First, ensure that your version of Vista has Aero. Home Premium, Business, Enterprise & Ultimate have Aero
Also ensure that your Graphic card supports WDDM (Windows Display Driver Model)
Next, right-clicking on the desktop, select Personalize
Click on Themes and select the default Windows Vista themes.
Next, in Windows Color & Appearance, ensure that Enable Transparency is checked.
Also ensure that 'Desktop Windows Manager' Service is set on Automatic.
Now, inspite of this, you still find that Aero does not work, or your Aero has stopped working, simply copy-paste the following command in Vista Start Menus Search Box, and hit Enter.