Aero doesn't work anymore

After a normal reboot, Aero disappeared from my HP Pavillion Dv 9500.
I didn't install anything new. To fix the problem I tried everything
from reindex the Windows Experience Index (it has not changed) to
check the DWM.
I tried a lot of fixes documented here:
http://www.microsoft.com/whdc/device...ero_rules.mspx

Unfortunately I can't set the Aero appearence and the preview panel
continue to be black:
http://faser.net/public/tmp/noaero.png

From the command line "winsat features" tell me that "DWM not running"
and in the event viewer I read that

The Desktop Window Manager was unable to start because a composited
theme is not in use
Event Id 9003

Unfortunately I can't find any fix to solve this problem.
Do you have any advice?