Aero has been disabled for win7 x64 since it was installed on my pc. The graphic card is Nvidia Geforce 7300. I updated the driver to the latest one, but still no lucky.
There is only one error in windows event log:

The Desktop Window Manager has encountered a fatal error (0x8007000e)

The error indicates ‘out of memory’. This make sense. I connected two monitors to the graphic card and enabled dual view by setting Multiple displays to ‘Extend these displays’ in display settings. And the ‘dual view’ consumes more video memory. (http://msdn.microsoft.com/en-us/library/ms800219.aspx)

After disabled the second monitor, Aero effect is back.

~~~~~~~~
Update:
After install a new graphic card, GT240, Aero effect can be enabled under ‘dual view’ mode now.