You can now easily identify causes due to which your Windows 7 is unable to turn on the Aero feature, and also fix them easily.
To do so, open Control Panel > All Control Panel Items > Troubleshooting > Appearance and Personalization
An dialog box to troubleshoot Aero will open. Click next.
The wizard will examine all possible causes like such as Video Memory, Desktop Windows Manager (DWM) service, theme service, power settings, color depth, etc, which could be creating this problem and present a report.
From here you will be able to see if all is OK or if there are any problems, they will be highlighted.
The wizard will also attempt to fix any issues.