Let's say you have already installed SharePoint 2010 on your server and you recently upgraded IIS7 / 7.5 to ASP.NET 4.0. Or you might changed the “SharePoint Central Administration V4” application pool’s .NET framework from version 2 to 4. You may get an error...
There is a duplicate 'system.web.extensions/scripting/scriptResourceHandler' section defined
...when you try to run central administration or any SharePoint application.
Check Solution at http://www.mssharepointtips.com/tip.asp?id=1172