If you are experiencing slow Windows 7 shutdowns, there is an easy way to identify the potential culprit (an antivirus program, driver, or other process).
Navigate to: Control Panel > All Control Panel Items > Performance Information and Tools > Advanced Tools. In some cases, this screen will display the problem files.
Failing that, open the “View Performance Details” option in the Event Log (Event Viewer). As described by BurrWalnut of the WindowsBBS forums, events in the 100 series are boot events and items in the 200 series are shutdown events. You will see an indicator of performance-slowing problems here.