When I start my Windows 7, I get a notification

The system event notification service could not be started....

As a result, I dont have Aero, even though my Windows Experience Index is 3.2.
Aero used to work fine before today and I have not installed any new application. I have checked the event log and it reads:

The System Event Notification Service service terminated with the following error:
Overlapped I/O operation is in progress.

I am not sure what this means.

I have seen the services.msc and everything is fine there. Moreover the startup time has also extended and the Please wait stays for more than a minute. This is a sudden problem as everything used to work fine until today.

For Aero I have tried starting Desktop Window manager (as pointed out by the trouble shooter), but with avail. From what I could understand from the forums, the lack of Aero and increased start-up time comes from a single problem of the System Event Notification Service.

This is a personal Computer and I have no active directories installed.