Problem Occur After Installing Umbraco4.7

Just two days ago, I decided to upgrade to IIS 7 so I'll be able to use .NET 4.0. Then I have installed Umbraco4.7, however I got the following error:

"Default Document
There was an error while performing this operation.
Details:
Line number:172
Error : The configuration section'system.web.extensions' cannot
be read because it is missing a declaration"
Please help me figure out how to fix it.
