Aero may be disabled if Windows thinks the system performance is too low to handle it. Maybe the update wiped your performance score. Any score of less than 3.0 will disable Aero by default, and below 2.0 you may not be able to force it.
Try refreshing the "experience index."
1.Open Performance Information and Tools by clicking the Start button , clicking Control Panel, clicking System and Maintenance, and then clicking Performance Information and Tools.
2.View the Windows Experience Index base score and subscores for your computer. If you have recently upgraded your hardware and want to find out if your score has changed, click Update my score. If you don't see subscores and a base score, click Score this computer.