Build website project in Visual Studio 2010 but showed error message of "Unrecognized attribute 'targetFramework'"

My original web site project is .NET2.0. However I upgraded the project to .NET4.0 today(using Visual Studio 2010 to transfer), but it showed the error message "Unrecognized attribute 'targetFramework'" and it directs to web.config ""

I have tried to run command " C:\WINDOWS\Microsoft.NET\Framework\v4.0.30319>aspnet_regiis -i ", also tried to upgrade IIS .netframework version to v4.0, there are still in vain. And also confirm my project's component are using .NET4.0.

Anyone could kindly help on this problem? Many thanks.