Was there any hardware change?
Here are some easy tasks to perform to enable Aero again:
1. Make sure that your Video driver is up-to-date. You may need to check with the manufacturer of the hardware's site to obtain their latest drivers that's compatible for your system.
2. If there was no hardware change and the video drivers are up to date, we may need to re-run the Windows Experience Index Assessment for enabling Aero.
a. To refresh the WEI (Windows Experience Index) Assessment Score, Click on Start.
b. Right Click Computer then select Properties, a new window with a System Information will be displayed.
c. Look for Rating and click the Windows Experience Index.
c. There should be a "Re-run the Assessment" near the bottom right side of window. NOTE: You may also access WEI through Control Panel>All Control Panel Items>Performance Information and Tools 3. Now click re-run the assessment. Wait for the result.
Once your Windows Experience Index is re-calculated and if its higher then 3.0, Aero in Windows 7 will be automatically enabled. You may now change to Aero Glass by right clicking on an empty space from the Desktop, choose Personalize and select Windows
Colors. If you still encounter problems you may run the FixIt tool from this link:http://support.microsoft.com/mats/aero_effects_not_working/en-us