There are times when the 'link' between IIS and ASP.NET can be broken, because of uninstall or reinstall of IIS or .NET environment.
In such as case U can use the Aspnet_regiis.exe tool that comes with the .NET SDK.
Just type : Aspnet_regiis.exe -r
Use Aspnet_regiis.exe -i if U also want to install the asp.net engine embedded within the regiis tool and update the IIS mapping.
Sometimes you may also have to register the Aspnet_isapi.dll by clicking start -> run.
In the Open text box, type “regsvr32 %windir%\Microsoft.NET\Framework\version\aspnet_isapi.dll” and then press ENTER