Error while installing the AJAXControl Toolkit

Question

Hello
 I am working with VS 2005 .
When I install the AJAXControlExtender.vsi , it installed successfully.But when I open a New Web site in VS 2005 I got the error
'Unknown Server Tag 'ajaxtoolkit:toolkitscriptmanager'

When I tried to install the AJAX Conrol toolkit in the toolbox, I got the error
' There was error loading types from assembly.Cound not find or file System.Web.Extensions.'

Can anyone provide the solution.

Thanks