HTTP Error 500.19 – Internal Server Error after installing Microsoft Office Web Apps on a SharePoint 2010 Server

After installing Microsoft Office Web Apps (following the steps on http://technet.microsoft.com/en-us/library/ff431687(office.14).aspx#bkmk_ins_exis_sa) on a Microsoft SharePoint 2010 server, I got a HTTP Error 500.19 when accessing the SharePiont Sites.   Error Config section ‘system.webServer/staticContent’ already defined.